Cyclic dipeptides. 3.1 Synthesis of methyl (R)-6-[(tert- butoxycarbonyl)amino]-4,5,6,7-tetrahydro-2-methyl-5-oxo-1,4-thiazepine-3- carboxylate and its hexahydro analogues: Elaboration of a novel dual ACE/NEP inhibitor

Article abstract of DOI:10.1021/jo981425v

Synthetic routes to highly functionalized 1,4-thiazepinones 2 and 3 have been developed. S-Ac-NBOC-L-Cys-D(L)-ThrOMe 7a,b have been prepared and, after transformation into the corresponding mesylates, used as the cyclization substrates. Treatment of these compounds with LiA1H(OMe)₃ in THF results in mesylate elimination and thiolacetate reduction, giving rise to both a Michael acceptor (α,β-unsaturated ester) and Michael donor (thiol anion), which undergo in situ intramolecular conjugate addition leading to the stereoselective formation of only two of the four possible stereoisomers of 2. On the other hand, PCC/CaCO₃ oxidation of 7a gave in 80% yield the corresponding ketone 11, which was in turn transformed into the enol triflate 15. Cleavage of the thiolacerate moiety, simultaneous elimination of trifluoromethanesulfonic acid to generate an allene system, and addition of the thiol group to the central carbon of the allene to provide the enantiomerically pure cyclic compound 3 in 85% yield was effected via a one- pot reaction by means of MeONa/MeOH. Thiazepinone 3 is an interesting intermediate for the preparation of conformationally restricted dipeptide mimetics, and its elaboration into the dual ACE/NEP inhibitor 4 is reported.

Full text of DOI:10.1021/jo981425v

J . Org. Chem. 1999, 64, 3019-3025
3019
Cyclic Dip ep tid es. 3.1 Syn th esis of Meth yl
(R)-6-[(ter t-Bu toxyca r bon yl)a m in o]-4,5,6,7-
tetr a h yd r o-2-m eth yl-5-oxo-1,4-th ia zep in e-3-ca r boxyla te
a n d Its Hexa h yd r o An a logu es: Ela bor a tion of a Novel Du a l
ACE/NEP In h ibitor †
Angela Crescenza, Maurizio Botta,* Federico Corelli,* Antonello Santini,‡ and Andrea Tafi
Dipartimento Farmaco Chimico Tecnologico, Universita` degli Studi di Siena, Banchi di Sotto 55,
53100 Siena, Italy, and Dipartimento di Scienza degli Alimenti, Universita` “Federico II”,
80055 Napoli, Italy
Received J uly 21, 1998 (Revised Manuscript Received J anuary 25, 1999)
Synthetic routes to highly functionalized 1,4-thiazepinones 2 and 3 have been developed. S-Ac-N-
Boc-L-Cys-D(L)-ThrOMe 7a ,b have been prepared and, after transformation into the corresponding
mesylates, used as the cyclization substrates. Treatment of these compounds with LiAlH(OMe)3 in
THF results in mesylate elimination and thiolacetate reduction, giving rise to both a Michael
acceptor (R,â-unsaturated ester) and Michael donor (thiol anion), which undergo in situ intra-
molecular conjugate addition leading to the stereoselective formation of only two of the four possible
stereoisomers of 2. On the other hand, PCC/CaCO3 oxidation of 7a gave in 80% yield the
corresponding ketone 11, which was in turn transformed into the enol triflate 15. Cleavage of the
thiolacetate moiety, simultaneous elimination of trifluoromethanesulfonic acid to generate an allene
system, and addition of the thiol group to the central carbon of the allene to provide the
enantiomerically pure cyclic compound 3 in 85% yield was effected via a one-pot reaction by means
of MeONa/MeOH. Thiazepinone 3 is an interesting intermediate for the preparation of conforma-
tionally restricted dipeptide mimetics, and its elaboration into the dual ACE/NEP inhibitor 4 is
reported.
In tr od u ction
tive heart failure (CHF).5 On the other hand, selective
NEP inhibitors have been recently shown to have diuretic
and natriuretic properties, without loss of potassium in
various experimental models of hypertension,6 but dis-
played hypotensive effects only in DOCA salt rats.7
Moreover, no significant reduction in arterial blood
pressure was observed in patients with CHF.8 Several
studies have shown that coadministration of selective
ACE and NEP inhibitors in animal models of hyperten-
sion and CHF has a more beneficial effect over the
administration of the single agents separately.9 In recent
reports, it has also been demonstrated that single mol-
ecules that possess dual ACE and NEP inhibitory activity
also exhibit these synergistic properties.10
The vasoactive peptides angiotensin-II (Ang-II) and
atrial natriuretic peptide (ANP) display opposing biologi-
cal effects. The former stimulates vasoconstriction and
sodium retention, while the latter produces vasodilation,
diuresis, and natriuresis and decreases the levels of
plasma renin and aldosterone.2 Therefore, a blockade of
Ang-II production concomitant with a potentiation of
endogenous ANP levels could represent a beneficial
approach to the treatment of various cardiovascular
disorders. The activity of these two antagonistic systems
is regulated essentially by metabolizing processes involv-
ing the zinc metallopeptidases, angiotensin-converting
enzyme (ACE, EC3.4.15.1), and neutral endopeptidase
(NEP, EC 3.4.24.11, neprilysin). ACE is responsible for
the maturation of Ang-II from its inactive precursor
angiotensin I (Ang-I), and NEP inactivates ANP.3 More-
over, both peptidases are involved in the metabolism of
bradykinin,4 a vasodilatory peptide. ACE inhibitors have
gained wide acceptance clinically and are commonly
prescribed for the treatment of hypertension and conges-
Conformationally restricted peptidomimetics have been
used extensively to probe the topography of enzyme
active sites and to generate potent inhibitors devoid of
the typical therapeutic shortcomings of peptides.11 Thi-
(5) (a) Lien, E. J .; Gao, H.; Lien, L. L. In Progress in Drug Research;
J ucker, E., Ed.; Birkhauser Verlag: Basel, 1994; Vol. 43, pp 43-86.
(b) Lawton, G.; Paciorek, P. M.; Waterfall, J . F. In Advances in Drug
Research; Testa, B., Meyer, U. A., Eds.; Academic Press: San Diego,
CA, 1992; Vol. 23, pp 161-220. (c) Waeshawsky, A. M.; Flynn, G. A.;
Koehl, J . R.; Mehdi, S. Bioorg. Med. Chem. Lett. 1996, 6, 957-962.
(6) Seymour, A. A.; Norman, J . A.; Asaad, M. M.; Fennell, S. A.;
Little, D. K.; Kratunis, V. J .; Rogers, W. L. J . Cardiovasc. Pharmacol.
1991, 17, 1025-1032.
(7) Pham, I.; El Amraani, A. I. K.; Fournie-Zaluski, M. C.; Corvol,
P.; Roques, B. P.; Michel, J . B. J . Cardiovasc. Pharmacol. 1992, 20,
847-857.
(8) Northridge, D. B.; J ardine, A. G.; Findlay, I. N.; Archibald, M.;
Dilly, S. G.; Dargie, H. J . Am. J . Hypertens. 1990, 3, 682-687.
(9) (a) Seymour, A. A.; Asaad, M. M.; Lanoce, V. M.; Langenbacher,
K. M.; Fennell, S. A.; Rogers, W. L. J . Pharmacol. Exp. Ther. 1993,
266, 872-883. (b) Pham, I.; Gonzales, W.; El Amraani, A. I. K.; Fournie-
Zaluski, M. C.; Philippe, M.; Laboulandine, I.; Roques, B. P.; Michel,
J . B. J . Pharmacol. Exp. Ther. 1993, 265, 1339-1347.
* To whom correspondence should be addressed. Tel: 39-577-45393.
Fax: 39-577-298183. E-mail: botta@unisi.it and corelli@unisi.it.
† Taken in part from: Crescenza, A. Ph.D. Thesis, University of
Siena, February 1998.
‡ Universita` “Federico II”.
(1) Part 2: Botta, M.; Crescenza, A.; Magara, W.; Corelli, F.
Tetrahedron Lett. 1997, 38, 2775-2778.
(2) (a) Brenner, B. M.; Ballerman, B. J .; Gunning, M. E.; Zeidel, M.
L. Physiol. Rev. 1990, 70, 665-700. (b) Winquist, R. J .; Hintze, T. H.
Pharmacol. Ther. 1990, 48, 417-426.
(3) Kenny, A. J .; Stephenson, S. L. FEBS Lett. 1988, 232, 507-513.
(4) Ura, N.; Carretero, O. A.; Erdos, E. G. Kidney Int. 1987, 32, 507-
513.
10.1021/jo981425v CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/02/1999

3020 J . Org. Chem., Vol. 64, No. 9, 1999
Crescenza et al.
Sch em e 1a
F igu r e 1.
azepinones and oxazepinones of general structure 1
(Figure 1) were studied by Bristol-Myers Squibb as dual
inhibitors, and compound 1a (X ) S; m, n ) 1; BMS-
186716), in particular, was advanced into clinical devel-
opment for the treatment of hypertension and CHF.12
Extension of our previous work on antihypertensive
agents13 has led us to identify the cyclic thiazepinones
2a ,b and 3 as interesting conformationally restricted
dipeptide mimetics en route to potential ACE/NEP dual
inhibitors such as 4. In this paper, we describe in full
the synthesis of 2a ,b14 and 31 as well as the further
elaboration of 3 into 4.
a
Reaction conditions: (a) D-threonine methyl ester (5a ), CMC,
HOBt, 92%; (b) MsCl, DIPEA, quant; (c) LiAlH(OMe)3, THF.
ertheless, recent strategies show that the cyclization is
possible in high yields, either by carbon-carbon17 or by
heteroatom-carbon18 bond formation.
To avoid the use of orthogonally N- and C-protected
amino acids, our approach to the target compounds
involves formation of the amide bond first, followed by
intramolecular displacement of a leaving group by a
sulfur nucleophile to obtain the heterocyclic ring. Reac-
tion between D-threonine methyl ester (5a )19 and S-
acetyl-N-tert-butoxycarbonyl-L-cysteine 6 (prepared20 from
N-tert-butoxycarbonyl-L-cystine) in the presence of N-cy-
clohexyl-N′-(2-morpholinoethyl)carbodiimide methyl p-
toluenesulfonate (CMC) and 1-hydroxybenzotriazole
(HOBt) afforded after chromatographic purification the
amide 7a in 92% yield as a single stereoisomer as
Resu lts a n d Discu ssion
Syn th esis of Hexa h yd r o-1,4-th ia zep in e Der iva -
tives 2a a n d 2b. It is well-known that cyclization of
linear precursors is in principle an excellent route to
heterocycles. However, for seven-membered rings, this
reaction is disfavored by entropic and enthalpic factors,15
so that these heterocycles are usually obtained in good
yield only when configurational and/or conformational
constraints facilitate intramolecular cyclization.16 Nev-
1
determined by TLC and H NMR spectroscopy (Scheme
1). Treatment of 7a with methanesulfonyl chloride and
N-ethyldiisopropylamine (DIPEA) at -5 °C gave the
corresponding mesylate 8a , which proved to be unstable
under the conditions of silica gel chromatography and
was therefore used in the next reaction step without
further purification. First attempts to convert 8a into the
cyclic derivatives 2 by a one-pot S-deacetylation-mesy-
late displacement sequence using methanolic ammonia
or sodium borohydride led to a complex mixture of
(10) (a) Trippodo, N. C.; Robl, J . A.; Asaad, M. M.; Bird, J . E.;
Panchal, B. C.; Schaeffer, T. R.; Fox, M.; Giancarli, M. R.; Cheung, H.
S. J . Pharmacol. Exp. Ther. 1995, 275, 745-752. (b) French, J . F.;
Anderson, B. A.; Downs, T. R.; Dage, R. C. J . Cardiovasc. Pharmacol.
1995, 26, 107-113. (c) Gonzales, W.; Fournie-Zaluski, M. C.; Pham,
I.; Laboulandine, I.; Roques, B. P.; Michel, J . B. J . Pharmacol. Exp.
Ther. 1995, 272, 343-351.
(11) Marshall, G. R. Tetrahedron 1993, 49, 3547-3558.
(12) Robl, J . A.; Sun, C.-Q.; Stevenson, J .; Ryono, D. E.; Simpkins,
L. M.; Cimarusti, M. P.; Dejneka, T.; Slusarchyk, W. A.; Chao, S.;
Stratton, L.; Misra, R. N.; Bednarz, M. S.; Asaad, M. M.; Cheung, H.
S.; Abboa-Offei, B. E.; Smith, P. L.; Mathers, P. D.; Fox, M.; Schaeffer,
T. R.; Seymour, A. A.; Trippodo, N. C. J . Med. Chem. 1997, 40, 1570-
1577
(13) (a) Corelli, F.; Manetti, F.; Tafi, A.; Campiani, G.; Nacci, V.;
Botta, M. J . Med. Chem. 1997, 40, 125-131. (b) Botta, B.; Misiti, D.;
Delle Monache, G.; Persichilli, S.; Vitali, A.; Botta, M.; Corelli, F.;
Carmignani, M. Gazz. Chim. Ital. 1997, 127, 305-310. (c) Corelli, F.;
Dei, D.; Delle Monache, G.; Botta, B.; De Luca, C.; Carmignani, M.;
Volpe, A. R.; Botta, M. Bioorg. Med. Chem. Lett. 1996, 6, 653-658. (d)
Delle Monache, G.; Botta, B.; Delle Monache, F.; Espinal, R.; De
Bonnevaux, S. C.; De Luca, C.; Botta, M.; Corelli, F.; Dei, D.; Gacs-
Baitz, E.; Carmignani, M. Bioorg. Med. Chem. Lett. 1996, 6, 233-238.
(e) Delle Monache, G.; Botta, B.; Delle Monache, F.; Espinal, R.; De
Bonnevaux, S. C.; De Luca, C.; Botta, M.; Corelli, F.; Carmignani, M.
J . Med. Chem. 1993, 36, 2956-2963. (f) Delle Monache, G.; Botta, B.;
Delle Monache, F.; Espinal, R.; De Bonnevaux, S. C.; De Luca, C.;
Botta, M.; Corelli, F.; Carmignani, M. Bioorg. Med. Chem. Lett. 1992,
2, 415-418.
(16) (a) Erickson, G. W.; Fry, J . L. J . Org. Chem. 1980, 45, 970-
972. (b) Massa, S.; Corelli, F.; Stefancich, G. J . Heterocycl. Chem. 1981,
18, 829-830. (c) Massa, S.; Stefancich, G.; Artico, M.; Corelli, F.;
Ortenzi, G. Heterocycles 1985, 23, 1417-1423.
(17) (a) Cockerill, G. S.; Kocienski, P.; Treadgold, R. J . Chem. Soc.,
Perkin Trans. 1 1985, 2093-2100. (b) Blumenkopf, T. A.; Overman,
L. E. Chem. Rev. 1986, 86, 857-873. (c) Castaneda, A.; Kucera, D. J .;
Overman, L. E. J . Org. Chem. 1989, 54, 5695-5707. (d) Coppi, L.; Ricci,
A.; Taddei, M. J . Org. Chem. 1988, 53, 911-913. (e) Yamamoto, Y.;
Yamada, J .; Kadota, I. Tetrahedron Lett. 1991, 32, 7069-7072.
(18) (a) Nicolau, K. C.; Hwang, C.-K.; Nugiel, D. A. J . Am. Chem.
Soc. 1989, 111, 4136-4137. (b) Kocienski, P. J .; Love, C. J .; Whitby,
R. J .; Costello, G.; Roberts, D. A. Tetrahedron 1989, 45, 3839-3848.
(c) Heslin, J . C.; Moody, C. J . J . Chem. Soc., Perkin Trans. 1 1988,
1417-1423. (d) Kotsuki, H.; Ushio, Y.; Kadota, I.; Ochi, M. J . Org.
Chem. 1989, 54, 5153-5161. (e) Nicolau, K. C.; Hwang, C.-K.; Duggan,
M. E.; Nugiel, D. A.; Abe, Y.; Bal Reddy, K.; DeFrees, S. A.; Reddy, D.
R.; Awartani, R. A.; Conley, S. R.; Rutjes, F. P. J . T.; Theodorakis, E.
A. J . Am. Chem. Soc. 1995, 117, 10227-10238.
(14) For a preliminary communication, see: Corelli, F.; Crescenza,
A.; Dei, D.; Taddei, M.; Botta, M. Tetrahedron: Asymmetry 1994, 5,
1469-1472.
(15) Illuminati, G.; Mandolini, L. Acc. Chem. Res. 1981, 14, 95-
102.
(19) Wieland, T.; Sarges, R. Ann. 1962, 658, 181-193.
(20) Zahn, H.; Hammerstroem, K. Chem. Ber. 1969, 102, 1048-1052.

Cyclic Peptides
J . Org. Chem., Vol. 64, No. 9, 1999 3021
Sch em e 2
Sch em e 3a
a
Reaction conditions: (a) PCC, CaCO3, CH2Cl2; (b) NH3,
products from which 2a was isolated in only very small
quantities after laborious chromatographic separations.
Subsequently, the deacetylation-cyclization reaction was
performed with lithium trimethoxyaluminum hydride (3
molar equiv) in dry THF. Under these conditions, com-
pounds 2a and 2b were obtained in 15% and 35% yield,
respectively, along with the olefins 9 (E/Z mixture, 20%)
and the disulfides 10 (12%). Bubbling nitrogen into the
reaction mixture suppressed the formation of 10 and led
to compounds 2a and 2b in a yield of 24% and 56%,
respectively, while the olefins 9 were always present in
trace amounts.
CH2Cl2; (c) (EtO)3CH, CSA, 4 Å MS.
8a , was cyclized by means of LiAlH(OMe)3, the same
diastereoisomers 2a and 2b were obtained (51% and 25%,
respectively), although the ratio of 2a /2b (2:1) in this case
was inverted with respect to the previous reaction (1:2).
Considering that olefin 9 might be a possible intermedi-
ate of the cyclization reaction, this compound (E/Z
mixture) was subjected to the cyclization under the same
experimental conditions as used for 8a , leading indeed
to the same products 2a and 2b in the ratio 2:1. This
same result was also obtained starting from 9 as a single
isomer22 (double-bond geometry not determined).
Assign m en t of Relative an d Absolu te Ster eoch em -
istr ies a n d Elu cid a tion of th e Cycliza tion Rea ction
P a th w a y. The structures of 2a and 2b were determined
In light of these results, we suggest that the cyclization
might occur through a preliminary methanesulfonic acid
elimination from 8a ,b to the olefin 9, followed by in-
tramolecular conjugate addition of the thiol group to give
2. A concurrent retro-Michael reaction might give rise
to an E/Z mixture of 9, which on cyclization leads to the
thermodynamically more stable isomers23 2a ,b via C-3
epimerization (Scheme 2). The obtaining of different 2a /
2b ratios starting from isomeric threonine mesylates 8a ,b
could be explained assuming that these compounds might
epimerize to a different extent prior to elimination,
leading to different ratios of (E/Z)-9.24
1
by FAB-MS and H NMR spectroscopy. The stereochem-
istry of the methyl at C-2 and the methoxycarbonyl group
at C-3 was proven to be cis for 2a by a nuclear Over-
hauser effect (NOE) between the C-2 and C-3 protons and
trans in the case of 2b (where no NOE was detected). To
establish the absolute stereochemistry at C-3, 2a was
desulfurated with Ra-Ni to methyl N-[N-(tert-butoxycar-
bonyl)-L-alanyl]-L-2-aminobutanoate,14 which proved to
be identical in all respects, including specific rotation,
with the material obtained from methyl (S)-(+)-2-ami-
nobutyrate21 and commercially available Boc-L-alanine.
These results show not only that the L-cysteinyl moiety
retains its configuration throughout the synthetic path-
way but also, and more interestingly, that both 2a and
2b possess a C-3 stereochemistry inverted with respect
to their precursor 8a .
Syn th esis of 1,4-Th ia zep in on e 3. Tetrahydrothiaz-
epinones of the type 3 were previously unknown, and we
envisioned that construction of the ring could be possible
via intramolecular addition of a thiol function to a ketone.
Accordingly, the dipeptide 7a (Scheme 3) was oxidized
25
with PCC/CaCO3 to give the corresponding ketone 11
in 80% yield. Other oxidizing reagents such as oxalyl
chloride/DMSO,26 tetrapropylammonium perruthenate/
(22) Prepared from 8a by reaction with potassium carbonate in
refluxing chloroform (see the Supporting Information).
(23) For molecular mechanics calculations on the four possible
stereoisomers of 2, see ref 14.
(24) We thank one of the reviewers for highlighting this point.
(25) Parish, E. J .; Luo, C.; Parish, S.; Heidepriem, R. W. Synth.
Commun. 1992, 22, 2839-2847.
(26) Mancuso, A. J .; Huang, S.-L.; Swern, D. J . Org. Chem. 1978,
43, 2480-2482.
When mesylate 8b, prepared from L-threonine methyl
ester and 6 (see Supporting Information) as described for
(21) Liu, W.; Ray, P.; Benezra, S. A. J . Chem. Soc., Perkin Trans. 1
1995, 553-559.

3022 J . Org. Chem., Vol. 64, No. 9, 1999
Crescenza et al.
was first described by Stang.34 The chemistry was
extended to the synthesis of allenylazetidinones by
Conway et al.35 and, more recently, by Kant and Farina,36
who demonstrated that these compounds are stable
enough to allow their isolation, chromatographic purifi-
cation, and storage.37 In view of this information, we
exposed 15 to CD3ONa in CD3OD, checking the reaction
by 1H NMR at 10 min intervals, and we noticed (i) a
prompt disappearance of the MeCOS signal; (ii) a steady
decrease in intensity of the Me-C-OTf signal until
complete disappearance; (iii) the appearance of a pair of
doublets at δ 4.57 and 4.54 with a coupling constant of
11.8 Hz, diagnostic of the cumulene system,38 whose
intensity decreased gradually; and (iv) an early (after the
first 10 min) appearance of the signal relative to the Me-
CdC of 3, whose intensity gradually increased during the
following 2 h. In view of this, we are led to conclude that
under the reported experimental conditions the trans-
formation of 15 to 3 most likely involves, though not
necessarily in this order, the following steps: (a) MeONa-
catalyzed transesterification of the thiolacetate, deliver-
ing the nucleophile (thiol anion); (b) formation of the
allene by elimination of methanesulfonic acid; and (c)
nucleophilic attack of the thiol anion to the central allenic
carbon, which is indeed susceptible to nucleophilic ad-
dition reaction when activated with an electron-with-
drawing group,39 leading to 7-endo-dig cyclization. It is
interesting to point out that attempts to cyclize an
analogue of 15, having a phenyl group instead of the
methyl and hence no possibility to generate an allene
intermediate, were unsuccessful.40
To definitely assess the intermediacy of an allenic
compound in the cyclization reaction, we subjected the
enol triflate 15 to the same reaction conditions reported
by Kant and Farina36 for the isolation of allenylazetidi-
nones, with the purpose to isolate the allene and to
transform it into 3 by means of MeONa. However,
isolation of this allene was prevented by its inherent high
reactivity, most likely due to the presence of a free NH
group on the cumulene system.
To establish its enantiomeric purity, 3 was oxidized
with m-CPBA to give only two diastereomeric sulfoxides
16a ,b, which by further oxidation led to the same
sulfone.14,41
Sch em e 4a
a
Reaction conditions: (a) Tf2O, DIPEA, CH2Cl2; (b) (EtO)3CH,
CSA, 4 Å MS; (c) MeONa, MeOH, -15 °C.
N-methylmorpholine N-oxide (TPAP/NMO),27 and PCC/
Al2O3 led to 11 in yields ranging between 24 and 42%.
28
The subsequent transformation of the thiolacetate group
into a free thiol proved to be troublesome, since, by
treatment of 11 with NH3, 12 was obtained in very low
yield (e10%), the main reaction product being the cor-
responding disulfide 13 (≈25-30%). On the other hand,
the use of other nucleophiles (MeONa, LiOH) led to
complex mixtures in which compounds deriving from
C-C bond cleavage at the ketone group were also
present. In an attempt to mask the ketone function as a
ketal, 11 was heated with triethyl orthoformate in the
presence of camphorsulfonic acid (CSA),29 and quite
unexpectedly, the disulfide 14 was isolated in 85% yield
after preparative TLC. This result clearly demonstrates
that the adopted conditions, though causing ketone
cleavage, nevertheless are able to efficiently remove the
sulfur protecting group, delivering a free thiol that is then
oxidized to the disulfide 14.
As a consequence of these findings, we sought a
different approach to 3. The ability of alkenyl triflates
to undergo solvolytic displacement through the inter-
mediacy of the corresponding vinyl cations is well docu-
mented in the literature.30 Therefore, we devised a new
avenue to our target molecule entailing the preparation
of enol triflate 15, followed by its intramolecular dis-
placement by an in situ generated thiol group (Scheme
4). To this end, ketone 11 was transformed by means of
triflic anhydride and DIPEA into the corresponding enol
1
triflate 15 in 88% yield. TLC and H NMR spectroscopy
revealed that only one isomer of 15 was obtained (double-
bond geometry not determined). By reaction with triethyl
orthoformate and CSA in refluxing methanol complete
conversion of substrate 15 was obtained in 6 h and 3 was
isolated in 52% yield after chromatographic purification.31
To improve the yield of the cyclization reaction, other
conditions were explored,32 and eventually it was found
that treatment of 15 with MeONa in dry MeOH at -15
°C for 2 h provided 3 in 85-90% yield.
Mech a n istic Con sid er a tion s a n d Assign m en t of
Absolu te a n d Rela tive Ster eoch em istr ies. The ob-
servation of facile allene formation from vinyl triflates33
The structure of 16b was confirmed by X-ray crystal-
lographic analysis (Figure 2), which showed that the
compound has the R configuration and also determined
(27) Ley, S. V.; Norman, J .; Griffith, W. P.; Marsden, S. P. Synthesis
1994, 639-666.
(28) Cheng, Y.-S.; Liu, W.-L.; Chen, S.-H. Synthesis 1980, 223-224.
(29) Meyers, A. I.; Temple, D. L.; Nolen, R. L.; Mihelich, E. D. J .
Org. Chem. 1974, 39, 2778-2783.
(30) (a) Summerville, R. H.; Schleyer, P. v. R. J . Am. Chem. Soc.
1972, 94, 3629-3631 and references therein. (b) Clarke, T. C.; Kelsey,
D. R.; Bergman, R. G. J . Am. Chem. Soc. 1972, 94, 3626-3629.
(31) No attempt has been made yet to assess the actual mechanism
of this cyclization reaction.
(34) Stang, P. J .; Hargrove, R. J . J . Org. Chem. 1975, 40, 657-658.
(35) Conway, T. T.; Lim, G.; Douglas, J . L.; Menard, M.; Doyle, T.
W.; Rivest, P.; Horning, D.; Morris, L. R.; Cimon, D. Can. J . Chem.
1978, 56, 1335-1341.
(36) Kant, J .; Roth, J . A.; Fuller, C. E.; Walker, D. G.; Benigni, D.
A.; Farina, V. J . Org. Chem. 1994, 59, 4956-4966 and references
therein.
(32) Attempts to cyclize 15 using LiAlH(OMe)3 as described for the
synthesis of 2 were unsuccessful, giving complex reaction mixtures in
which thiazepine 3 was never detected.
(33) For a recent review on vinyl and aryl triflates, see: Ritter, K.
Synthesis 1993, 735-761.
(37) They also suggested that the formation of allenylazetidinones
would occur through a mechanism of the E1cb type. See also: (a)
Hansen, R. L. J . Org. Chem. 1965, 30, 4322-4324. (b) Streitwieser,
A., J r.; Wilkins, C. L.; Kiehlmann, E. J . Am. Chem. Soc. 1968, 90,
1598-1601.

Cyclic Peptides
J . Org. Chem., Vol. 64, No. 9, 1999 3023
Attempts to hydrolyze both the ester groups at the same
time were unsatisfactory, as complex reaction mixtures
were obtained; therefore, we opted for the selective
cleavage of each of them. The use of LiI in pyridine
allowed for the selective cleavage of the methyl ester
function and the acid 21 was obtained in 66% yield along
with 25% recovered 20. Finally, reaction of 21 with
MeONa/MeOH led to the target compound 4 in 50% yield
after careful chromatographic purification. The biological
evaluation of 4 as a dual ACE/NEP inhibitor is still
ongoing, and the results will be reported in due course.
Con clu sion s
We have reported in detail the straightforward syn-
thesis of two conformationally restricted dipeptides 2a
and 2b through a three-step sequence involving (a) amide
bond formation between readily available, suitably pro-
tected, L-cysteine and D- or L-threonine, (b) mesylation
of a secondary hydroxy group, and (c) LiAlH(OMe)3-
mediated elimination of methanesulfonic acid and reduc-
tion of the thiolacetate function delivering both the
nucleophile (thiol anion) and electrophile (R,â-unsatur-
ated ester), which undergo intramolecular conjugate
addition. This synthesis is also stereoselective in that it
provides only two of the possible stereoisomers of methyl
6-[(tert-butoxycarbonyl)amino]hexahydro-2-methyl-5-oxo-
1,4-thiazepine-3-carboxylate in chiral nonracemic form
starting from either D- or L-threonine, with the product
ratios depending on the absolute configuration of the
starting amino acids. Moreover, a concise synthesis of
enantiomerically pure cyclic dipeptide 3 has been ac-
complished in four steps starting from readily available,
protected amino acids. The key step is based on a one-
pot cyclization procedure entailing the MeONa-mediated
cleavage of a thiolacetate moiety and triflic acid elimina-
tion to an intermediate allene, which undergoes intra-
molecular nucleophilic addition in situ, with formation
of a new S-C bond leading to the cyclization product.
Compound 3 is an interesting synthon for the preparation
of dual ACE/NEP inhibitors such as 4, as well as of other
peptidomimetics based on a conformationally restricted
dipeptide scaffold.
F igu r e 2. Stereoview of the X-ray structure of 16b.
Sch em e 5a
a
Reaction conditions: (a) TFA, CH2Cl2; (b) 18, CMC, HOBt,
CH2Cl2; (c) CsSAc, DMF; (d) Lil, Py; (e) MeONa, MeOH.
the relative stereochemistry of the substituents, with the
side chain at C-6 and oxygen at sulfur being cis. Hence,
the diastereomeric sulfoxide 16a was assigned the 1S-
trans configuration.
Exp er im en ta l Section
Ela bor a tion of 3 in to th e Du a l In h ibitor 4. The
transformation of 3 into 4 entails deprotection and
subsequent acylation of the amino group at C-6, followed
by hydrolysis of the ester function to deliver the free acid.
To this end, 3 was treated with TFA in CH2Cl2 (Scheme
5) to give the amine 17, which was subjected to acylation
with (R)-2-bromo-3-phenylpropionic acid (18)42 in the
presence of CMC and HOBt to provide the amide 19 in a
most satisfactory yield of 54%, considering that, due to
their low stability on silica gel, both 17 and 18 were used
in this reaction without previous purification. On treat-
ment with cesium thiolacetate43 in DMF, 19 underwent
a clean displacement of bromide with complete inversion
of configuration giving 20 (70%) as a sole diastereomer.
Gen er a l Meth od s. All moisture-sensitive reactions were
performed under an argon atmosphere using oven-dried
glassware. All solvents were dried over standard drying
agents44 and freshly distilled prior to use. Reagents were from
commercial suppliers and used without further purification.
Extracts were dried over Na2SO4 and evaporated under
reduced pressure with a rotary evaporator. Merck silica gel
60 was used for chromatography (70-230 mesh) and flash
chromatography (230-400 mesh) columns. Analytical and
preparative TLC were performed with Merck silica gel 60 F254
(0.2 and 2 mm thickness, respectively) precoated aluminum
sheets with visualization by UV light, charring with H2SO4
(10% in water), or charring with anisaldehyde in ethanolic
sulfuric acid.45 Melting points are uncorrected. Optical rota-
tions were measured at 20 ( 2 °C in CHCl3 unless otherwise
(38) Allred, E. L.; Grant, D. M.; Goodlett, W. J . Am. Chem. Soc. 1965,
87, 673-674.
(43) (a) Flynn, G. A.; Beight, D. W.; Mehdi, S.; Koehl, J . R.; Giroux,
E. L.; French, J . F.; Hake, P. W.; Dage, R. C. J . Med. Chem. 1993, 36,
2420-2423. (b) Strijveen, B.; Kellogg, R. M. J . Org. Chem. 1986, 51,
3664-3671.
(39) Schuster, H. F.; Coppola, G. M. Allenes in Organic Synthesis;
J ohn Wiley & Sons: New York, 1984; Chapter 6.
(40) Crescenza, A. Ph.D. Thesis, University of Siena, 1998.
(41) Compounds 16b and the corresponding sulfone exhibited in
vitro cytotoxic activity in the MTT test at 50 µM concentration.
(42) Briggs, M. T.; Morley, J . S. J . Chem. Soc., Perkin Trans. 1 1978,
2138-2143.
(44) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.
(45) Casey, M.; Leonard, J .; Lygo, B.; Procter, G. Advanced Practical
Organic Chemistry, 1st ed.; Chapman & Hall: New York, 1990.

3024 J . Org. Chem., Vol. 64, No. 9, 1999
Crescenza et al.
stated. IR spectra were recorded in CHCl3 solutions (unless
otherwise stated). 1H NMR spectra were measured at 200
MHz. Chemical shifts are reported relative to CDCl3 at δ 7.24
ppm and tetramethylsilane at δ 0.00 ppm. EI and FAB low-
resolution mass spectra were recorded with an electron beam
of 70 eV. Elemental analyses (C, H, N) were performed in-
house.
2b: 35% yield; yellowish oil; Rf 0.38 (EtOAc/hexanes 1.5:1);
1H NMR (CDCl3) δ 6.61 (d, 1H, J ) 8.0 Hz), 5.91 (d, 1H, J )
5.0 Hz), 4.50 (m, 1H), 4.20 (m, 1H), 3.75 (s, 3H), 3.38 (m, 1H),
2.65 (d, 2H, J ) 9.1 Hz), 1.41 (s, 9H), 1.40 (d, 3H, J ) 7.2 Hz);
IR 3420, 2990, 1750, 1680 cm-1; FABMS (TDEG-GLY) m/z 341
(M + Na) +, 319 (M + H)+. Anal. Calcd for C13H22N2O5S: C,
49.04; H, 6.96; N, 8.80. Found: C, 49.28; H, 7.17; N, 8.66.
When the same reaction was performed by bubbling argon
into the reaction mixture, compounds 2a and 2b were obtained
in 24 and 56% yield, respectively, while compound 9 was
present only in trace amount.
N-[S-Acetyl-N-(ter t-bu toxycar bon yl)-L-cystein yl]-D-th r e-
on in e Meth yl Ester (7a ). A solution of HOBt (1.37 g, 10.2
mmol) in dry dichloromethane (20 mL) was added to a cold
(0-5 °C) solution of 5a (1.70 g, 12.7 mmol) and 6 (2.22 g, 8.5
mmol) in the same solvent (100 mL). After 5 min, a solution
of CMC (5.30 g, 12.7 mmol) in dichloromethane (30 mL) was
added dropwise, and the mixture was kept at room tempera-
ture for 4 h. The solution was washed successively with 1 N
HCl, aqueous NaHCO3 (5% solution), and brine and then dried,
and the solvent was evaporated in vacuo. The residue was
purified by column chromatography with EtOAc to give 7a
(2.94 g, 92%) as a solid: mp 121-123 °C (from toluene/
cyclohexane, 1:1); [R]D -10 (c 1.0); 1H NMR (CDCl3) δ 7.12 (d,
1H, J ) 8.6 Hz), 5.32 (d, 1H, J ) 7.1 Hz), 4.60 (dd, 1H, J )
8.8 and 2.6 Hz), 4.41 (m, 1H + 1H), 3.77 (s, 3H), 3.27 (m, 2H),
2.40 (s, 3H), 2.30 (d, 1H), 1.45 (s, 9H), 1.21 (d, 3H, J ) 6.6
Hz); IR 3420, 2970, 1690 cm-1. Anal. Calcd for C15H26N2O7S:
C, 47.61; H, 6.92; N, 7.40. Found: C, 47.72; H, 7.05; N, 7.36.
N-[S-Acetyl-N-(ter t-bu toxycar bon yl)-L-cystein yl]-D-th r e-
on in e Meth yl Ester , Meth a n esu lfon a te (8a ). A solution of
methanesulfonyl chloride (0.10 mL, 1.4 mmol) in dry dichlo-
romethane (2 mL) was added dropwise to a cold (-5 °C)
solution of 7a (0.50 g, 1.3 mmol) and DIPEA (0.26 mL, 1.5
mmol) in the same solvent (20 mL). After being stirred at room
temperature for 14 h, the solution was cooled at -5 °C, and
further amounts of DIPEA (0.52 mL, 3.0 mmol) and methane-
sulfonyl chloride (0.10 mL, 1.4 mmol), each diluted in dichlo-
romethane (2 mL), were added successively. After 30 min, the
solution was diluted with cold water and neutralized with solid
NaHCO3. The organic layer was washed with brine, dried, and
concentrated to a yellow oil (0.60 g, 100%) homogeneous by
TLC: 1H NMR (CDCl3) δ 7.06 (d, 1H), 5.29 (m, 1H + 1H), 4.80
(dd, 1H), 4.36 (m, 1H), 3.79 (s, 3H), 3.30 (m, 2H), 3.01 (s, 3H),
2.39 (s, 3H), 1.48 (s, 9H), 1.41 (d, 3H).
Con ver sion of 8a in to 2a ,b, 9, a n d 10. A solution of 8a
(0.60 g, 1.3 mmol) in dry THF (10 mL) was added dropwise to
a solution of LiAlH(OMe)3 prepared by adding dry MeOH (0.63
mL, 15.6 mmol) to a suspension of LiAlH4 (197 mg, 5.2 mmol)
in dry and degassed THF (20 mL). After the solution was
stirred for 2 h at room temperature, the excess reducing agent
was decomposed with acetone (0.5 mL) and then water (0.5
mL). The reaction mixture was neutralized with citric acid,
and most of the THF was removed under reduced pressure.
EtOAc was added to the residue, and the organic solution was
washed with brine, dried, and evaporated. Chromatography
on silica gel (EtOAc/hexanes 1.5:1) afforded the following
compounds:
2a : 15% yield; white solid; mp 147-149 °C; Rf 0.74 (EtOAc/
hexanes 1.5:1); [R]D -15 (c 1.4); 1H NMR (CDCl3) δ 6.50 (d,
1H, J ) 5.2 Hz), 6.01 (d, 1H, J ) 5.1 Hz), 4.85 (d, 1H, J ) 5.1
Hz), 4.60 (m, 1H), 3.80 (s, 3H), 3.31 (q, 1H, J ) 7.2 Hz), 2.90
(dq, 2H, J ) 9.9, and 2.0 Hz), 1.50 (s, 9H), 1.20 (d, 3H, J ) 7.2
Hz); IR 3410, 3000, 1730, 1710, 1680 cm-1; FABMS (TDEG-
GLY) m/z 319 (M + H)+. Anal. Calcd for C13H22N2O5S: C,
49.04; H, 6.96; N, 8.80. Found: C, 49.18; H, 7.12; N, 8.84.
Meth yl N-[S-Acetyl-N-(ter t-bu toxyca r bon yl)-L-cystein -
yl]-2-a m in o-3-oxobu ta n oa te (11). CaCO3 (0.86 g, 8.52 mmol)
and 4 Å MS (both dried at 250 °C overnight) were added to a
solution of 7a (0.65 g, 1.7 mmol) in dry CH2Cl2 (200 mL). After
the mixture was stirred at room temperature for 15 min, PCC
(1.51 g, 7.0 mmol) was added, and the mixture was left stirring
at the same temperature as other portions of PCC were added
five times at 1 h intervals to achieve a reagent/substrate ratio
of 24.7:1. After 18 h, a further portion of PCC was added, and
1 h later the reaction mixture was filtered through Celite and
then through a short silica gel column (CH2Cl2/EtOAc 10:1).
Evaporation of eluates afforded a residue that was purified
by column chromatography (SiO2-EtOAc/hexanes 1.5:1) to
yield 7a (0.52 g, 80%) as white crystals: mp 103-105 °C (from
EtOAc); [R]D +10.1 (c 1.0); 1H NMR (CDCl3) δ 7.43 (1H, s),
5.29-5.23 (1H, m), 5.20 (1H, dd, J ) 7.1, 5.4 Hz), 4.39 (1H,
m), 3.82 (3H, s), 3.41 (1H, dd, J ) 14.4, 4.7 Hz), 3,21 (1H, dd,
J ) 14.4, 7.1 Hz), 2.38 (3H + 3H, s), 1.46 (9H, s); IR (Nujol)
3324, 2923, 2853, 1748, 1727, 1691, 1653, 1559, 1521 cm-1
.
Anal. Calcd for C15H24N2O7S: C, 47.86; H, 6.43; N, 7.45.
Found: C, 47.65; H, 6.33; N, 7.50.
Meth yl N-[S-Acetyl-N-(ter t-bu toxyca r bon yl)-L-cystei-
n yl]-2-a m in o-3-t r iflu or om et h a n esu lfon yloxy-2-b u t en o-
a te (15). TEA (519 µL, 3.98 mmol) and DMAP (cat.) were
added to a solution of 11 (0.50 g, 1.33 mmol) in dry CH2Cl2
(100 mL), and the reaction mixture was stirred for 1 h at room
temperature and then cooled to -78 °C. Tf2O (447 µL, 2.65
mmol) was added over 2 min, and after 15 min at -78 °C the
mixture was warmed to room temperature and washed suc-
cessively with 1 N HCl, water, and brine. The organic layer
was dried and evaporated, and the residue was purified by
column chromatography on silica gel (CH2Cl2/EtOAc 10:1) to
give 15 (0.62 g, 92%) as a yellowish solid: mp 129-132 °C;
[R]D -31.3 (c 1.0); 1H NMR (CDCl3) δ 8.10 (1H, s), 5.21 (1H, d,
J ) 7.4 Hz), 4.41 (1H, m, J ) 4.6, 7.9 Hz), 3.82 (3H, s), 3.38
(1H, dd, J ) 14.6, 4.6 Hz), 3.22 (1H, dd, J ) 14.6, 7.9 Hz),
2.45 (3H, s), 2.38 (3H, s), 1.46 (9H, s); IR 3400, 3030, 2980,
1730, 1710, 1695 cm-1. Anal. Calcd for C16H23F3N2O9S2: C,
37.79; H, 4.56; N, 5.51. Found C, 37.55; H, 4.42; N 5.38.
Meth yl (R)-6-[(ter t-Bu toxyca r bon yl)a m in o]-4,5,6,7-tet-
r a h yd r o-2-m eth yl-5-oxo-1,4-th ia zep in e-3-ca r boxyla te (3).
Meth od A. A solution of 15 (0.05 g, 0.10 mmol) in dry MeOH
(10 mL) was reacted with triethyl orthoformate (0.03 g, 0.20
mmol) in the presence of CSA (cat.) and 4 Å molecular sieves.
After the solution was heated at reflux for 6 h, solid NaHCO3
(0.05 g) was added to the cooled (5 °C) mixture. Filtration and
evaporation afforded a residue that was taken up into EtOAc,
and this solution was washed with aqueous 5% NaHCO3
solution and brine, dried, and evaporated. Purification of the
crude product by silica gel chromatography (CHCl3/Et2O 15:
1) provided 3 (0.016 g, 52%) as a yellowish solid: mp 152-
154 °C (from CHCl3/Et2O); [R]D -20.7 (c 0.8); 1H NMR (CDCl3)
δ 7.25 (1H, s), 5.70 (1H, d, J ) 6.0 Hz), 4.56 (1H, m), 3.73 (3H,
s), 3.39 (1H, dd, J ) 11.4, 11.0 Hz), 3.12 (1H, dd, J ) 11.4, 3.2
Hz), 2.35 (3H, s), 1.36 (9H, s); IR 3412, 3378, 1690 cm-1; EIMS
m/z 316 (M+); HRMS calcd for C13H20N2O5S 316.1093, found
316.1096. Anal. Calcd for C13H20N2O5S: C, 49.37; H, 6.33; N,
8.86. Found: C, 49.42; H, 6.35; N, 8.79.
9 (E/Z mixture): 20% yield; Rf 0.58 (EtOAc/hexanes 1.5:1);
1H NMR (CDCl3) δ 7.83 (s, 1H), 6.76 (q, 1H), 5.43 (d, 0.5H),
5.39 (d, 0.5H), 4.36 (m, 1H), 3.69 (s, 3H), 3.27 (2dd, 1H), 2.86
(d, 1H), 2.31 (s, 1.5H), 2.11 (s, 1.5H), 1.70 (2d, 3H), 1.39 (s,
9H). Anal. Calcd for C15H24N2O6S: C, 49.99; H, 6.71; N, 7.77.
Found: C, 50.21; H, 6.84; N, 7.60.
10: 12% yield; Rf 0.45 (EtOAc/hexanes 1.5:1); 1H NMR
(CDCl3) δ 8.88 (s, 2H), 6.89 (q, 2H), 5.63 (d, 2H), 5.13 (m, 2H),
3.79 (s, 6H), 3.12 (2dd, 4H), 1.75 (d, 6H), 1.38 (s, 18H); IR 3440,
3320, 3000, 1740, 1510 cm-1; FABMS (TDEG-GLY) m/z 635
(M + H)+. Anal. Calcd for C26H42N4O10S2: C, 49.21; H, 6.62;
N, 8.83. Found: C, 49.17; H, 6.70; N, 8.97.
Meth od B. To a solution of 15 (0.54 g, 1.06 mmol) in dry
MeOH (160 mL) maintained at -15 °C was added MeONa
(0.086 g, 1.59 mmol). After being stirred for 2 h at this
temperature, the reaction mixture was gradually warmed to
room temperature, neutralized with citric acid, and concen-
trated. The residue was dissolved in EtOAc, and this solution

Cyclic Peptides
J . Org. Chem., Vol. 64, No. 9, 1999 3025
was washed with water, dried, and evaporated. Purification
of the reaction product as reported in method A gave 3 (0.29
g, 85%), whose analytical and spectroscopic data are identical
to those of the compound obtained by method A.
boxylic Acid (4). MeONa (0.024 g, 0.32 mmol) was added to
a cooled (0 °C) solution of 21 (0.070 g, 0.17 mmol) in dry MeOH
(2 mL). After being stirred for 4 h, the reaction mixture was
acidified with 1 N HCl and concentrated in vacuo at room
temperature. The residue was dissolved in EtOAc, and the
resultant solution was dried and evaporated. Purification of
the crude product by silica gel chromatography (EtOAc/AcOH
9:1) provided 4 (0.031 g, 50%) as a foam: mp 241-244 °C dec;
Meth yl (R)-6-Am in o-4,5,6,7-tetr a h yd r o-2-m eth yl-5-oxo-
1,4-th ia zep in e-3-ca r boxyla te (17). A solution of 3 (0.050 g,
0.16 mmol) in dry CH2Cl2 (1.0 mL) was stirred at 0 °C as a
50% solution of TFA in CH2Cl2 (1.0 mL) was added over 5 min.
After 2 h, the reaction mixture was made alkaline by addition
of solid NaHCO3 and filtered. Evaporation of volatiles afforded
a residue (0.034 g) that proved to be unstable on silica gel and
was therefore used in the next step without purification.
Meth yl [R-(R*,R*)]-6-[(2-Br om o-1-oxo-3-p h en ylp r op yl)-
a m in o]-4,5,6,7-tetr a h yd r o-2-m eth yl-5-oxo-1,4-th ia zep in e-
3-ca r boxyla te (19). A solution of crude 17 (0.066 g, 0.31
mmol) and 1842 (0.058 g, 0.25 mmol) in the same solvent (5
mL) was reacted following the same procedure reported for
the preparation of 7a . Column chromatography on silica gel
(EtOAc/hexanes 1.5:1) of the reaction product provided 19
(0.058 g, 54%): [R]D -7.7 (c 0.7); 1H NMR (CDCl3) δ 7.55 (1H,
s), 7.35 (1H, d, J ) 6.2 Hz), 7.25-7.19 (5H, m), 4.76 (1H, m),
4.42 (1H, t, J ) 7.5 Hz), 3.76 (3H, s), 3.53-2.87 (2H + 2H, m),
2.41 (3H, s); FABMS (TDEG-GLY) m/z 427 (M+). Anal. Calcd
for C17H19BrN2O4S: C, 47.78; H, 4.45; N, 6.56. Found: C,
47.49; H, 4.66; N, 6.47.
Meth yl [S-(R*,S*)]-6-[(2-Acetylth io-1-oxo-3-p h en ylp r o-
p yl)a m in o]-4,5,6,7-t et r a h yd r o-2-m et h yl-5-oxo-1,4-t h ia z-
ep in e-3-ca r boxyla te (20). Compound 19 (0.065 g, 0.15 mmol)
was dissolved in a 3 M solution of CH3COSCs in DMF43b (0.08
mL, 024 mmol), and the reaction mixture was left at room
temperature for 24 h and then diluted with Et2O (10 mL). This
cloudy solution was washed with water (5 × 5 mL), dried, and
evaporated. Purification of the crude product by chromatog-
raphy on silica gel (EtOAc/hexanes 1:1) gave 20 (0.045 g, 70%)
as a sole diastereoisomer: 1H NMR (CDCl3) δ 7.42 (1H, s),
7.30-7.14 (5H + 1H, m), 4.68 (1H, m), 4.29 (1H, t, J ) 7.4
Hz), 3.76 (3H, s), 3.42-2.89 (2H + 2H, m), 2.40 (3H, s), 2.29
(3H, s); IR 3200, 3100, 1692 cm-1; FABMS (TDEG-GLY) m/z
422 (M+). Anal. Calcd for C19H22N2O5S2: C, 54.03; H, 5.21; N,
6.64. Found: C, 54.30; H, 5.33; N, 6.48.
[S-(R*,S*)]-6-[(2-Acetylth io-1-oxo-3-p h en ylp r op yl)a m i-
n o]-4,5,6,7-t et r a h yd r o-2-m et h yl-5-oxo-1,4-t h ia zep in e-3-
ca r boxylic Acid (21). LiI (0.060 g, 0.48 mmol) was added to
a solution of 20 (0.100 g, 0.24 mmol) in dry pyridine (4.0 mL),
and the resultant mixture was refluxed for 2 h. After cooling,
the solution was diluted with water (15 mL) and EtOAc (15
mL) and acidified with 1 N HCl. The organic layer was
separated, washed with water, dried, and evaporated. The
residue was purified by passing through a silica gel column.
Elution with EtOAc afforded 0.031 g (30%) of the starting
compound 20, while further elution with EtOAc/AcOH (95:5)
gave 21 (0.064 g, 66%) as a foam: 1H NMR (CDCl3 + D2O) δ
7.27-7.23 (5H, m), 4.78 (1H, m), 4.27 (1H, m), 3.36-2.90 (2H
+ 2H, m), 2.43 (3H, s), 2.30 (3H, s); IR 3400, 3250, 1740, 1680
cm-1; FABMS (TDEG-GLY) m/z 408 (M+). Anal. Calcd for
1
[R]D -10.7 (c 1.0); H NMR (CDCl3 + D2O) δ 7.49-7.21 (5H,
m), 4.61-440 (1H + 1H, m), 3.34-2.71 (2H + 2H, m), 2.32
(3H, s); IR 3410, 3250, 2200, 1745, 1690, cm-1; FABMS (TDEG-
GLY) m/z 365 (M+). Anal. Calcd for C16H18N2O4S2: C, 52.46;
H, 4.92; N, 7.65. Found: C, 52.80; H, 4.75; N, 7.46.
Cr ysta l Da ta of th e Su lfoxid e 16b. Solu tion a n d r e-
fin em en t: C13H20N2O6S, M ) 332.4, monoclinic, space group
P21, a ) 5.962(2) Å, b ) 10.281(8) Å, c ) 13.949(7) Å, â )
101.19(7)°, V ) 838.8 Å3, T ) 23 °C, Z ) 2, Dc ) 1.316 g/cm3,
monochromated Cu KR radiation, λ ) 1.5418 Å. Data were
collected on a four-circle Enraf-Nonius CAD-4 diffractometer
using ω - 2θ scan. A total of 1684 reflections were collected,
of which 1684 were unique. The structure was solved by direct
methods (SIR 92)46 and refined on F2 by full-matrix least-
squares procedures using the SDP package47 to give R ) 0.049
and Rw ) 0.045 for 1022 observed independent reflections with
I > 3σ(I). Non-hydrogen atoms were made anisotropic, and
hydrogen atoms were included in calculated positions. All
calculations were performed on a VAX3100 Digital computer
of the Biocrystallography Research Centre of CNR at the
Department of Chemistry, University of Naples “Federico II”.48
Ack n ow led gm en t. Thanks are due to Dr. D. J abe´s,
Immunology Department, Pharmacia & Upjohn, Milano,
Italy, for performing biological tests. Financial support
from Italian CNR (Grant No. 97.02708.CT03) is grate-
fully acknowledged. One of us (M.B.) wishes to thank
the Merck Research Laboratories for the 1998 Academic
Development Program (ADP) Chemistry Award. A.S.
gratefully acknowledges Mr. Maurizio Muselli and Mrs.
Gabriella De Vita for their technical assistance and the
financial support of the CNR and the Ministry of
Education of Italy (40% and 60%).
Su p p or tin g In for m a tion Ava ila ble: X-ray data for com-
pound 16b and experimental procedures and characterization
data for 8a , 9 (single isomer), 12-14, and 16a ,b. This material
is available free of charge via the Internet at http://pubs.acs.org.
J O981425V
(46) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994, 27,
435-472.
(47) SDP, Structure Determination Package, Enraf-Nonius, Delft,
The Netherlands, 1982.
(48) The authors have deposited atomic coordinates for this struc-
ture with the Cambridge Crystallographic Data Centre. The coordi-
nates can be obtained, on request, from the Director, Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ,
U.K.
C
18H20N2O5S2: C, 52.94; H, 4.90; N, 6.86. Found: C, 53.19; H,
4.77; N, 7.06.
[S-(R*,S*)]-6-[(2-Mer capto-1-oxo-3-ph en ylpr opyl)am in o]-
4,5,6,7-t et r a h yd r o-2-m et h yl-5-oxo-1,4-t h ia zep in e-3-ca r -