Rapid Regio- and Diastereoselective Paterno - Buechi Reaction of Alkyl Phenylglyoxylates

Article abstract of DOI:10.1021/jo9615054

Triplet excited states of alkyl phenylglyoxylates react rapidly (k -9.4 × 10⁹ M^-1 s^-1) with electron rich alkenes forming oxetanes with high regio and stereoselectivity. The well-known intramolecular γ-hydrogen abstraction (Norrish type II) cannot compete. When less electron-rich alkenes are used, the Norrish type II reaction becomes competitive. Intramolecular Paterno-Buechi reactions predominate in those alkyl phenylglyoxylates containing properly situated electron-rich alkene groups. The regioselectivity of this reaction is explained by the stability of the intermediate 1,4-biradical. The appropriate conformation at the instant of intersystem crossing determines the stereoselectivity of the products. The priority of the Paterno-Buechi over the Norrish type II reaction is understood by considering the conformation of phenylglyoxylate esters.

Full text of DOI:10.1021/jo9615054

564
J . Org. Chem. 1997, 62, 564-567
Ra p id Regio- a n d Dia ster eoselective P a ter n o`-Bu1 ch i Rea ction of
Alk yl P h en ylglyoxyla tes
Shengkui Hu and Douglas C. Neckers*
Center for Photochemical Sciences,1 Bowling Green State University, Bowling Green, Ohio 43403
Received August 5, 1996X
Triplet excited states of alkyl phenylglyoxylates react rapidly (k ) 9.4 × 109 M-1 s-1) with electron
rich alkenes forming oxetanes with high regio and stereoselectivity. The well-known intramolecular
γ-hydrogen abstraction (Norrish type II) cannot compete. When less electron-rich alkenes are used,
the Norrish type II reaction becomes competitive. Intramolecular Paterno`-Bu¨chi reactions
predominate in those alkyl phenylglyoxylates containing properly situated electron-rich alkene
groups. The regioselectivity of this reaction is explained by the stability of the intermediate 1,4-
biradical. The appropriate conformation at the instant of intersystem crossing determines the
stereoselectivity of the products. The priority of the Paterno`-Bu¨chi over the Norrish type II reaction
is understood by considering the conformation of phenylglyoxylate esters.
Sch em e 1
In tr od u ction
[2 + 2] Photocycloaddition of carbonyl compounds to
ethylenes yielding oxetanes (Paterno`-Bu¨chi reaction) is
well known.2 In our recent reinvestigation of the pho-
tochemical reactions of alkyl phenylglyoxylates,3 we
found that methyl phenylglyoxylate undergoes a rapid
Paterno`-Bu¨chi reaction (k ) 9.4 × 109 M-1 s-1) with high
chemical yield (92%). We have now carried out a more
complete study on the Paterno`-Bu¨chi reaction of phen-
ylglyoxylates and on its competition with the well-
understood Norrish type II reaction.
Resu lts a n d Discu ssion
The Paterno`-Bu¨chi reaction has been widely used in
synthetic organic chemistry to prepare oxetane-contain-
ing chiral natural products or as a route to photoaldoliza-
tion.4 Scharf and co-workers used the Paterno`-Bu¨chi
reaction of chiral alkyl phenylglyoxylates for the deter-
mination of isoinversion points.5 In the study we report
herein, we have carried out both inter- and intramolecu-
lar Paterno`-Bu¨chi reactions of alkyl phenylglyoxylates
and observed the interplay of Paterno`-Bu¨chi and Nor-
rish type II reactions when different alkenes are used.
Ethyl phenylglyoxylate (1) was irradiated in the pres-
ence of an equimolar amount of a series of alkenes in
benzene solution (0.01 M). Two different reactions
proceed depending on the structure of the alkene used;
see Scheme 1. Pathway A is the Paterno`-Bu¨chi cycload-
dition reaction forming oxetanes and occurs rapidly with
electron-rich alkenes. Pathway B is the Norrish type II
photolysis of 1 wherein the alkene is recovered un-
changed. The results of reactions using different alkenes
are accumulated in Table 1.
We intentionally varied the structure of the alkenes
used in this study from more to less electron rich (Table
1). With electron-rich alkenes (enol ethers or polyalkyl-
substituted alkenes 2-8), the Paterno`-Bu¨chi reaction
dominates, and no products from Norrish type II reac-
tions are observed. With monosubstituted alkenes (10,
11), no Paterno`-Bu¨chi reaction is observed. Further
reducing the electron richness of the alkenes (alkenes
12-15) produces similar results. This confirms that the
S1 state of ethyl phenylglyoxylate is too short lived to
react via a Paterno`-Bu¨chi pathway. The regio- and
stereoselectivity displayed in the Paterno`-Bu¨chi reaction
X Abstract published in Advance ACS Abstracts, J anuary 15, 1997.
(1) Contribution No. 284 from the Center for Photochemical Sci-
ences.
(2) For reviews, see: (a) Arnold, D. R. Adv. Photochem. 1968, 6, 301-
423. (b) J ones, G., II. Org. Photochem. 1981, 5, 1-122. (c) Carless, H.
A. J . In Synthetic Organic Photochemistry; Horspool, W. M., Ed.;
Plenum: New York, 1984; pp 425-487. (d) Porco, J . A., J r.; Schreiber,
S. L. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I.,
Paquette, L. A., Eds.; Plenum: New York, 1991; Vol. 5, pp 151-192.
(e) Griesbeck, A. G. In Handbook of Photochemistry and Photobiology;
Horspool, W. A., Song, P.-S., Eds.; CRC Press: Boca Raton, 1994. (f)
Turro, N. J . Modern Molecular Photochemistry; University Book: 1991;
pp 432-452. (g) Neckers, D. C. Mechanistic Organic Photochemistry;
Reinhold: New York, 1967; pp 131-138.
(3) (a) Huyser, E. S.; Neckers, D. C. J . Org. Chem. 1964, 29, 276-
278. (b) Hu, S.; Neckers, D. C. J . Org. Chem. 1996, 61, 6407-6415.
(4) (a) Mattay, J .; Buchkremer, K. Helv. Chim. Acta 1988, 71, 981-
987. (b) Mattay, J .; Buchkremer, K. Heterocycles 1988, 27, 2153-2165.
(c) Araki, Y. J . Chem. Soc., Perkin Trans. 1 1981, 12. (d) Schreiber, S.
L. Science 1985, 227, 857-863. (e) Schreiber, S. L. J . Am. Chem. Soc.
1983, 105, 660-661. (f) Schreiber, S. L.; Satake, K. J . Am. Chem. Soc.
1984, 106, 4186-4188. (g) Schreiber, S. L.; Hoveyda, A. H. J . Am.
Chem. Soc. 1984, 106, 7200-7202. (h) Nehrings, A.; Scharf, H.-D.;
Runsink, J . Angew. Chem. 1985, 97, 882. Angew Chem., Int. Ed. Engl.
1985, 24, 877-879. (i) Weuthen, M.; Scharf, H.-D.; Runsink, J .; Vassen,
R. Chem. Ber. 1988, 121, 971-976. (j) Kozluk, T.; Zamojski, A.
Tetrahedron 1983, 39, 805-810.
1
of phenylglyoxylate is pure T (n,π*)-state photochemis-
try. Alkene 9 is unreactive in the Paterno`-Bu¨chi reac-
tion contrary to 6-8 because 9 is more flexible. Besides
its electron richness, structural rigidity influences the
alkene’s reactivity in the Paterno`-Bu¨chi reaction.
Alkyl phenylglyoxylates with internal double bonds, 23
and 24, were synthesized and irradiated in benzene
solution (0.01 M). Compound 25 and its photochemistry
have been reported.6 These results are collected in Table
2.
(5) (a) Buschmann, H.; Scharf, H.-D.; Hoffmann, N.; Esser. P.
Angew. Chem. 1991, 103, 480; Angew Chem., Int. Ed. Engl. 1991, 30,
477-515. (b) Buschmann, H.; Scharf, H.-D.; Hoffmann, N.; Plath, M.
W.; Runsink, J . J . Am. Chem. Soc. 1989, 111, 5367-5373.
(6) Kraus, G. A.; Wu, Y. J . Am. Chem. Soc. 1992, 114, 8705-8707.
S0022-3263(96)01505-8 CCC: $14.00 © 1997 American Chemical Society

Paterno-Bu¨chi Reaction of Alkyl Phenylglyoxylates
J . Org. Chem., Vol. 62, No. 3, 1997 565
Ta ble 1. Resu lts of Eth yl P h en ylglyoxyla te Rea ction s
w ith Differ en t Alk en es
Sch em e 2
tion playing a major role. No intermolecular Paterno`-
Bu¨chi reaction is observed for 24. The intramolecular
Paterno`-Bu¨chi reaction is also absent for compound 25
since its alkene functionality is not sufficiently electron
rich.
The major products of the Paterno`-Bu¨chi reactions
(Tables 1 and 2) show both high regio- and stereoselec-
tivity. In most cases, regioselectivities are higher than
95%. This can be understood by considering the mech-
anism of the Paterno`-Bu¨chi reaction as shown in Scheme
2. The intermediate 1,4-biradical was detected by pico-
second absorption spectroscopy.7 When the alkene used
is asymmetric, the most stable 1,4-biradical (3D rather
3
than D′) is formed to furnish major products regioselec-
tively.
The stereoselectivity of the Paterno`-Bu¨chi reaction is
attributed to the conformational memory effects during
the intersystem crossing (ISC) of the triplet 1,4-biradical.8
Because of the relatively long lifetime of the triplet
biradicals as compared to singlet biradicals, a decrease
in stereoselectivity should be observed in photoreactions
via triplet biradicals. On the other hand, spin-orbit
coupling (SOC),9 the dominating factor that determines
lifetime of the 1,4-biradical, is strongly dependent on the
geometry of the triplet biradical. The triplet 1,4-biradical
has to assume a specific conformation before ISC. Be-
cause of the very short lifetime of the resulting singlet
1,4-biradical, the appropriate ISC conformation is re-
flected in the stereoselectivity of photoproducts.
The Salem-Rowland rules10 governing the mecha-
nisms of SOC require the conservation of total angular
momentum. This can be achieved when the axes of the
p orbitals at the radical centers are oriented perpendicu-
lar to each other. A plausible explanation for the
stereoselectivity of product formation is depicted in
Scheme 3. Two conformations of the triplet radical (3DI
Ta ble 2. Alk yl P h en ylglyoxyla tes w ith In ter n a l Dou ble
Bon d a n d Th eir P h otor ea ction Resu lts
3
and DII) can fulfill the Salem-Rowland requirement and
1
are able to ISC to singlet states (1DI and DII). 1DI and
1DII can dissociate to the starting materials or cyclize
3
forming oxetanes. Triplet DII is less stable due to steric
(7) Freilich, S. C.; Peters, K. S. J . Am. Chem. Soc. 1985, 107, 3819-
3822.
Compound 23 does not undergo a Norrish type II
reaction since no γ-hydrogen is available, neither does it
undergo a Paterno`-Bu¨chi reaction (the alkene portion
is insufficiently electron rich and the two functional
groups are too close together). Compound 24 undergoes
both the Norrish type II process and the intramolecular
Paterno`-Bu¨chi reaction with the Paterno`-Bu¨chi reac-
(8) Griesbeck, A. G.; Mauder, H.; Stadtmu¨ller, S. Acc. Chem. Res.
1994, 27, 70-75.
(9) For a review, see: Michl, J .; Klessinger, M. Excited States and
Photochemistry of Organic Molecules; VCH: New York, 1995; pp 219-
228.
(10) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11,
92.
(11) Griesbeck, A. G.; Stadtmu¨ller, S. J . Am. Chem. Soc. 1990, 112,
1281.

566 J . Org. Chem., Vol. 62, No. 3, 1997
Hu and Neckers
Sch em e 3
Sch em e 4
Therefore, the Norrish type II reaction is only possible
in the excited state of the E conformer, which is populated
by only about 1%18 of the molecules in solution. This
explains why the Norrish type II rate constant of phen-
ylglyoxylates is 2 orders of magnitude smaller than that
of similar phenyl ketones.3b Since Paterno`-Bu¨chi reac-
tions can occur in both E and Z conformers, the low
population of the E conformer does not effect the rate of
the Paterno`-Bu¨chi reaction.
In summary, phenylglyoxylates react with electron-rich
alkenes forming oxetanes with predictable stereochem-
istry. The priority of the Paterno`-Bu¨chi reaction over
Norrish type II reaction can be explained by the unfavor-
able Norrish type II conformation of phenylglyoxylates.
repulsion11 and is thus depopulated. The lifetimes of 1DI
and DII are too short to allow significant rotation around
1
the C-O bond. Thus, the proper conformations assumed
by the triplets (3DI,3DII) are preserved in the photoprod-
ucts. This allows the prediction of the conformations of
the products in Tables 1 and 2. Semiempirical calcula-
tions (AM1) show that the major products are about
0.5-1 kcal/mol more unstable than the corresponding
minor products. These results show that the stereose-
lectivity observed is contrathermodynamic control and
SOC-controlled spin inversion is the essential factor
determining the stereochemistry of products.
In general, when an electron-rich alkene is present,
the triplets of the alkyl phenylglyoxylate undergo the
Paterno`-Bu¨chi reaction in lieu of the Norrish type II
photolysis. The reaction rate of the Norrish type II
reaction for methyl phenylglyoxylate is estimated to be
at least 4 orders of magnitude lower than that of its
photocycloaddition with alkene 1.3b The low reaction rate
of the Norrish type II of methyl phenylglyoxylate is not
successfully attributed to either the nature of the γ-hy-
drogen or the geometry of the transition state.12 We
found, however, that the low rate of the Norrish type II
reaction of phenylglyoxylate esters can be successfully
attributed to the conformation of these R-keto esters. In
principle, normal esters might exist in either E or Z
conformations.13 Z conformers are found to be 5-6 kcal/
mol more stable than the corresponding E conformers.14
We performed geometry optimization on ethyl phenylg-
lyoxylate15 using the AM1 semiemperical method16 and
found that the E conformer, which exists in the required
geometry for intramolecular hydrogen abstraction,17 is
4.37 kcal/mol higher in energy than the Z conformer
(Scheme 4).
Exp er im en ta l Section
Ma ter ia ls. Benzene (Aldrich) was dried over sodium under
argon. Ethyl phenylglyoxylate (1) was obtained from Aldrich
and purified by column chromatography before use. Other
chemicals were used as received. NMR spectra were taken
with either a Varian Gemini 200 NMR spectrometer or a
Varian Unity Plus 400 NMR spectrometer. Chemical shifts
are in ppm with TMS as the internal standard. GC/MS were
taken on a Hewlett-Packard 5988 mass spectrometer coupled
to a HP 5880A GC, interfaced to a HP 2623A data processor.
Infrared spectra were taken with a Galaxy series 6020 FTIR
spectrometer. Thin-layer chromatography was performed with
Whatman silica gel coating TLC plates. Products are isolated
by flash column chromatography using Aldrich silica gel (60
Å, 70-270 mesh). The elution solvents were indicated follow-
ing the compound name. Semiempirical calculations were
performed with a Silicon Graphics Indigo using the package
Spartan.19 HRMS were obtained from the University of Illinois
at Urbana-Champaign. General procedures for irradiating
samples and isolating photoreaction products were reported
earlier.3b
2-(E t h oxyca r b on yl)-2-p h en yl-3,3,4,4-t et r a m et h ylox-
eta n e (16) (m ixtu r e of tw o en a n tiom er s): hexanes:ethyl
acetate ) 20:1; slightly yellow liquid; 1H NMR (400 MHz,
CDCl3) δ 0.86 (s, 3H), 1.26 (s, 3H), 1.29 (t, J ) 7.6 Hz, 3H),
1.33 (s, 3H), 1.45 (s, 3H), 4.15-4.31 (m, 2H), 7.25-7.40 (m,
3H), 7.55-7.70 (m, 2H); 13C NMR (50 MHz, CDCl3) δ 14.17,
22.22, 22.45, 25.44, 25.60, 47.01, 61.03, 85.08, 88.30, 126.08,
127.32, 127.54, 139.30, 172.72; MS 77 (12), 84 (15), 105 (100),
158 (5.1), 189 (22), 204 (1.3, M - 58); IR (neat) 2971.29,
2928.84, 1744.19, 1724.69, 1481.79, 1447.06, 1373.74; HRMS
m/e calcd 262.1566, found 262.1569.
(12) Pappas, S. P.; Alexander, J . E.; Zehr, R. D., J r. J . Am. Chem.
Soc. 1970, 92, 6927-6931.
(13) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of
Organic Compounds; J ohn Wiley & Sons, Inc.: New York, 1994;
Chapter 10.
(14) (a) Blom, C. E.; Gu¨nthard, H. H. Chem. Phys. Lett. 1981, 84,
267-271. (b) Ruschin, S.; Bauer, S. H. J . Phys. Chem. 1980, 84, 3061-
3065. (c) Wiberg, K. B.; Laidig, K. E. J . Am. Chem. Soc. 1987, 109,
5935-5943. (d) J ones, G. I. L.; Owen, N. L. J . Mol. Struct. 1973, 18, 1.
(e) Allinger, N. L.; Chang, S. H. M. Tetrahedron 1977, 33, 1561. (f)
Radom, L.; Lathan, W. A.; Hetre, W. J .; Pople, J . A. Austr. J . Chem.
1972, 25, 1601.
2-(E t h oxyca r b on yl)-2-p h en yl-3,3,4-t r im et h yloxet a n e
(17) (m ixtu r e of tw o en a n tiom er s): hexanes:ethyl acetate
1
) 20:1; clear oil; H NMR (200 MHz, CDCl3) δ 0.78 (s, 3H),
1.23 (d, J ) 6.6 Hz, 3H), 1.30 (s, 3H), 1.30 (t, J ) 7.0 Hz, 3H),
4.25 (dd, J ) 7.0 Hz, 1.4 Hz, 2H), 4.68 (q, J ) 6.6 Hz, 1H),
7.20-7.40 (m, 3H), 7.50-7.63 (m, 2H); 13C NMR (50 MHz,
CDCl3) δ 14.24, 17.06, 19.70, 25.39, 45.97, 61.24, 83.55, 90.00,
125.92, 127.32, 127.37, 138.78, 172.27; MS 77 (44.5), 91 (5.1),
105 (100), 158 (3.6), 175 (21.3), 179 (19.6), 204 (1.0, M - 44);
HRMS m/e calcd 248.1413, found 248.1412.
(15) All 14 alkyl phenylglyoxylates involved in the earlier study (ref
3b) have been studied. Similar results are obtained.
(16) AM1 was chosen because it gives the most stable conformer
structure in excellent agreement with single-crystal X-ray diffraction
of phenylglyoxylate. Runsink, J .; Koch, H.; Nehrings, A.; Scharf, H.-
D.; Nowack, E.; Hahn, T. J . Chem. Soc., Perkin Trans. 2 1988, 49-55.
(17) Wagner, P. J .; Meador, M. A.; Zhou, B.; Park, B-S. J . Am. Chem.
Soc. 1991, 113, 9630-9639. Wagner, P. J .; Zhou, B.; Hasegawa, T.;
Ward, D. L. J . Am. Chem. Soc. 1991, 113, 9640-9654. Gudmundsdot-
tir, A. D.; Lewis, T. J .; Randall, L. H.; Scheffer, J . R.; Rettig, S. J .;
Trotter, J .; Wu, C.-H. J . Am. Chem. Soc. 1996, 118, 6167-6184.
en d o-8-(E t h oxyca r b on yl)-8-p h en yl-2,7-d ioxa b icyclo-
[4.2.0]octa n e (18) (m ixtu r e of tw o en a n tiom er s): hexanes:
(18) Obtained by taking the energy difference in solution is half of
that in gaseous phase (Grindley, T. B. Tetrahedron Lett. 1982, 23, 1757)
and utilizing Boltzmann distribution law, XE/XZ ) exp(∆H/RT).
(19) Spartan version 4.0.1, Wavefunction Inc., 18401 Von Karman
Ave., Suite 370, Irvine, CA 92715.

Paterno-Bu¨chi Reaction of Alkyl Phenylglyoxylates
J . Org. Chem., Vol. 62, No. 3, 1997 567
ethyl acetate ) 20:1-5:1; clear oil; 1H NMR (200 MHz, CDCl3)
δ 1.29 (t, J ) 6.8 Hz, 3H), 1.76-1.88 (m, 2H), 1.94-2.06 (m,
2H), 3.81 (s, 1H), 3.94 (t, J ) 5.8 Hz, 2H), 4.20-4.32 (m, 2H),
6.21 (s, 1H), 7.24-7.38 (m, 3H), 7.52-7.61 (m, 2H); 13C NMR
(50 MHz, CDCl3) δ 20.30, 21.97, 65.47, 80.28, 98.40, 114.48,
126.91, 127.76, 127.87, 139.62, 143.94, 174.22; MS 55 (11.1),
77 (17.3), 84 (100), 91 (3.2), 105 (34.9), 143 (1.2), 189 (14.1, M
- 73); HRMS m/e calcd 262.1204, found 262.1205.
2-(E t h oxyca r b on yl)-2-p h en yl-3-(2′-ch lor oet h oxyl)ox-
eta n e (19) (m ixtu r e of tw o en a n tiom er s): hexanes:ethyl
acetate ) 1:2; yellowish oil; 1H NMR (200 MHz, CDCl3) δ 1.29
(t, J ) 7.2 Hz, 3H), 3.32 (m, 2H), 3.66 (t, J ) 5.2Hz, 2H), 4.28
(q, J ) 7.2Hz, 2H), 4.49 (t, J ) 7.0 Hz, 1H), 4.76 (t, J ) 7.0
Hz, 1H), 4.92 (t, J ) 7.0 Hz, 1H), 7.22-7.39 (m, 3H), 7.48-
7.63 (m, 2H); 13C NMR (50 MHz, CDCl3) δ 14.01, 42.14, 63.93,
70.07, 73.57, 77.99, 92.98, 125.78, 128.25, 128.45, 134.43,
171.79; MS 43 (23.3), 63 (19.9), 77 (26.8), 105 (100), 106 (60.4),
108 (17.6), 146 (5.9), 204 (4.6), 254 (1.8, M - 30); HRMS m/e
calcd 284.0817, found 284.0815.
en d o-8-(Eth oxyca r bon yl)-8-p h en yl-7-oxa bicyclo[4.2.0]-
oct-2-en e (20) (m ixtu r e of tw o en a n tiom er s): hexanes:
ethyl acetate ) 10:1; yellowish oil; 1H NMR (400 MHz, CDCl3)
δ 1.27 (t, J ) 6.8 Hz, 3H), 1.92-2.15 (m, 4H), 3.82 (t, J ) 11.2
Hz, 1H), 4.24 (m, 2H), 5.28 (m, 1H), 5.50 (m, 1H), 5.82 (m,
1H), 7.25 (m, 1H), 7.32 (m, 2H), 7.37 (m, 2H); 13C NMR (50
MHz, CDCl3) δ 14.07, 19.92, 25.19, 41.19, 61.73, 75.69, 89.28,
123.24, 125.09, 126.90, 127.21, 127.39, 127.70, 131.70, 137.71,
173.65; MS 51 (13.2), 77 (58.4), 79 (56.9), 80 (100), 105 (70.3),
2878.68, 1728.75, 1447.06, 1242.54, 1080.47, 1022.58; MS 51
(4.3), 67 (5.5), 77 (20.9), 91 (3.1), 105 (100), 115 (7.4), 173 (30.0),
174 (4.1, M - 72); HRMS calcd for C12H13O 173.0966, found
173.0966.
1′,1′-Dim eth yl-2′-p r op en yl Ben zoylfor m a te (23). Ben-
zoylformyl chloride was synthesized as reported.20 Equimolar
amounts of 2-methyl-3-buten-2-ol and benzoylformyl chloride
were dissolved in chloroform. Triethylamine was added drop-
wise over 1 h. The resulting solution was stirred for 4 h and
washed with saturated NaHCO3 solution and water each for
three times. The title compound was obtained in 82% yield
after column chromatography (hexanes:ethyl acetate ) 20:1):
1H NMR (400 MHz, CDCl3) δ 1.78 (m, 3H), 1.80 (m, 3H), 4.89
(dt, J ) 7.2, 0.4 Hz, 2H), 5.47 (tq, J ) 7.2, 0.4 Hz, 1H), 7.50
(m, 2H), 7.63 (m, 1H), 7.99 (m, 2H); 13C NMR (50 MHz, CDCl3)
δ 18.12, 25.77, 62.95, 117.30, 128.82, 129.99, 132.50, 134.79,
141.11, 163.83, 186.39; MS 41 (44.8), 69 (67.4), 77 (63.5), 105
(100), 218 (0.01, M); HRMS m/e calcd 218.0943, found 218.0944.
3′,7′-Dim eth yl-6′-octen yl Ben zoylfor m a te (24). The title
compound was synthesized by the DCC reaction described
earlier:3b 1H NMR (400 MHz, CDCl3) δ 0.96 (d, J ) 6.4 Hz,
3H), 1.19-1.27 (m, 2H), 1.35-1.41 (m, 1H), 1.59 (s, 3H), 1.66
(s, 3H), 1.80-1.86 (m, 2H), 1.95-2.04 (m, 2H), 4.44 (t, J ) 5.8
Hz, 2H), 5.08 (tq, J ) 8.0, 1.6 Hz, 1H), 7.49 (t, J ) 7.2 Hz,
2H), 7.64 (t, J ) 7.2 Hz, 1H), 8.01 (d, J ) 7.2 Hz, 2H); 13C
NMR (50 MHz, CDCl3) δ 17.57, 19.27, 25.28, 25.62, 29.32,
35.20, 36.83, 64.73, 124.31, 128.81, 129.92, 131.40, 132.43,
134.79, 163.91, 186.37; MS 41 (24.1), 56 (29.3), 77 (31.4), 95
(15.8), 105 (100), 123 (10.0), 138 (8.7), 155 (3.4), 224 (8.1), 270
(2.3), 288 (0.3, M); HRMS m/e calcd 288.1725, found 288.1724.
180 (15.1), 185 (6.2, M - 73); HRMS m/e calcd for C13H13
185.0966, found 185.0966.
O
6,10,10-Tr im et h yl-2-oxo-1-p h en yl-3,11-d ioxa b icyclo-
[7.1.1]n on a d eca n e (26) (m ixtu r e of tw o en a n tiom er s):
hexanes:ethyl acetate ) 20:1; 1H NMR (400 MHz, CDCl3) δ
0.92 (d, J ) 6.8 Hz, 3H), 1.24 (s, 3H), 1.37 (m, 1H), 1.40 (s,
3H), 1.59 (m, 2H), 1.67 (m, 2H), 2.13 (m, 2H), 2.92 (dd, J )
1.2Hz, 0.4Hz, 1H), 3.78 (m, 1H), 4.77 (m, 1H), 7.29 (m, 2H),
7.37 (m, 2H), 7.54 (m, 1H); 13C NMR (50 MHz, CDCl3) δ 17.82,
21.07, 23.88, 30.23, 30.49, 30.74, 31.41, 52.63, 66.13, 82.52,
84.16, 125.94, 127.33, 127.89, 141.91, 171.46; MS 77 (16.3),
105 (100), 159(13.2), 244 (3.3), 288 (0.5, M); HRMS m/e calcd
288.1725, found 288.1727.
en d o-8-(Eth oxyca r bon yl)-8-p h en yl-7-oxa bicyclo[4.2.0]-
octa n e (21) (m ixtu r e of tw o en a n tiom er s): hexanes:ethyl
acetate ) 10:1; yellowish oil; 1H NMR (400 MHz, CDCl3) δ 0.88
(t, J ) 6.4 Hz, 2H), 1.04 (m, 2H), 1.25 (t, J ) 7.2 Hz, 3H), 1.40
(m, 2H), 1.59 (m, 2H), 3.33 (dt, J ) 6.4, 8.4 Hz, 1H), 4.23 (m,
2H), 5.01 (m, 1H), 7.29 (m, 1H), 7.36 (m, 2H), 7.45 (m, 2H);
13C NMR (100 MHz, CDCl3) δ 14.09, 19.07, 20.33, 28.10, 31.57,
40.75, 61.60, 75.38, 87.67, 125.31, 127.53, 128.04, 137.83,
174.22; IR (neat) 3003.90, 2936.56, 2870.96, 1728.75, 1450.92,
1257.97, 1.34.16; MS 51 (2.6), 77 (15.8), 91 (2.8), 105 (100),
187 (23.5), 214 (0.1, M - 46); HRMS calcd for C13H15
O
187.1123, found 187.1122.
en d o-7-(Eth oxyca r bon yl)-7-p h en yl-6-oxa bicyclo[3.2.0]-
h ep ta n e (22) (m ixtu r e of tw o en a n tiom er s): petroleum
ether:ethyl acetate ) 10:1; yellowish oil; H NMR (400 MHz,
CDCl3) δ 1.24 (t, J ) 7.2 Hz, 3H), 1.04 (m, 2H), 1.31 (m, 2H),
1.43 (m, 1H), 1.51 (m, 1H), 1.64 (m, 1H), 2.00 (m, 1H), 3.59
(dt, J ) 5.2, 0.8 Hz, 1H), 4.21 (m, 2H), 5.35 (m, 1H), 7.29 (m,
1H), 7.36 (m, 2H), 7.42 (m, 2H); 13C NMR (100 MHz, CDCl3)
δ 14.02, 24.07, 27.07, 33.65, 47.02, 61.60, 84.20, 86.10, 125.32,
127.42, 127.80, 136.88, 174.04; IR (neat) 3060.04, 2959.71,
Ack n ow led gm en t. We thank the National Science
Foundation (Grant Nos. DMR-9013109 and CHE-
9302619) and the Office of Naval Research (Grant No.
N00014-91-J -1921) for financial support.
1
Su p p or tin g In for m a tion Ava ila ble: NMR spectra for
16-24 and 26 (12 pages). This material is contained in
libraries on microfiche, immediately follows this article in the
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ACS; see any current masthead page for ordering information.
(20) Ottenheijm, H. C. J .; De Man, J . H. M. Synthesis 1975, 163-
164.
J O9615054