Stereoselective synthesis of 1-methylcarbapenem precursors: Studies on the diastereoselective hydroformylation of 4-vinyl β-lactam with aminophosphonite-phosphinite and aminophosphine-phosphite rhodium(I) complexes

Article abstract of DOI:10.1016/j.tetasy.2004.10.021

The asymmetric hydroformylation of variously N-substituted 4-vinyl β-lactams catalyzed by rhodium aminophosphonite-phosphinite and rhodium aminophosphine-phosphite complexes was studied. These products are valuable intermediates in the preparation of 1-me

Full text of DOI:10.1016/j.tetasy.2004.10.021

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 3841–3845
Stereoselective synthesis of 1-methylcarbapenem precursors:
studies on the diastereoselective hydroformylation of 4-vinyl
b-lactam with aminophosphonite–phosphinite and
aminophosphine–phosphite rhodium(I) complexes
Edoardo Cesarotti* and Isabella Rimoldi
`
Universita degli Studi di Milano, Facolta di Farmacia, Dipartimento di Chimica Inorganica,
Metallorganica e Analitica e Istituto CNR-ISTM, Via G. Venezian, 21, I-20133 Milano, Italy
`
Received 29 July 2004; accepted 25 October 2004
Abstract—The asymmetric hydroformylation of variously N-substituted 4-vinyl b-lactams catalyzed by rhodium aminophosphon-
ite–phosphinite and rhodium aminophosphine–phosphite complexes was studied. These products are valuable intermediates in the
preparation of 1-methylcarbapenem antibiotics; the stereoselectivity to the desired b-isomer is related to the presence of a substituent
at the N atom of the b-lactam ring. The regioselectivity (branched/linear) but not the stereoselectivity (b/a) was found to be depend-
ent on the substrate to catalyst ratio.
Ó 2004 Published by Elsevier Ltd.
1. Introduction
developing synthetic routes to the 1b-methylcarbapenem
1, in particular, to the key intermediate, monocyclic b-
lactam 3.
1b-Methylcarbapenem 1 and the other homologous b-
lactams are unnatural antibiotics1 that have stimulated
considerable interest due to their dehydropeptidase sta-
bility and to the improved chemical stability compared
to that of the founder of the family, the potent broad
spectrum antibiotic thyenamicin 2, a fungal metabolite
discovered in the late 70s2 (Scheme 1).
An elegant way to obtain 3 is based on the highly regio-
selective and diastereoselective hydroformylation of the
4-vinyl b-lactam, (3S,4R)-3-[(R)-1-(tert-butyldimethyl-
silyloxy)ethyl-4-vinyl-2-azetidinone] 4a to give 5ab fol-
lowed by the oxidation of the branched b-aldehyde;
the reaction was first reported by Nozaki et al.4 using
Rh(acac)(CO)2 and the chiral bidentate phosphorus lig-
and BINAPHOS5 (Scheme 2).
OH
O
OH
O
OH
O
+
NH3
S
SR
COOH
COOH
Alper et al.,6 in 1999, obtained a higher diastereo- and
regioselectivity using the catalyst derived from the zwit-
terionic rhodium complex, (NBD)Rh+B(Ph)4 and the
chiral phosphine (S,S)-2,4-bis(diphenylphosphino)pen-
tane, (S,S)-BDPP, a ligand originally prepared by Bos-
nichin teh 80s and successfully used in asymmetric
hydrogenations.7
N
N
NH
COO-
1
2
3
Scheme 1.
Due to the lack of practical biotechnological methods
of preparation,3 many efforts have been made in
Recently we have prepared new electron deficient
aminophosphonite–phosphinite and aminophosphine–
phosphite ligands, which gave satisfactory Rh(I)-cata-
lysts for asymmetric hydrogenations of the dehydro-
aminoacids.8 These ligands, as described in Scheme 3,
combine the chirality of one stereogenic sp3 carbon
*
Corresponding author. Tel.: +39 02 503 14390; fax: +39 02 503
14405; e-mail: edoardo.cesarotti@unimi.it
0957-4166/$ - see front matter Ó 2004 Published by Elsevier Ltd.
doi:10.1016/j.tetasy.2004.10.021

3842
E. Cesarotti, I. Rimoldi / Tetrahedron: Asymmetry 15 (2004) 3841–3845
OTBS
OTBS
O
OTBS
OTBS
CHO
Rh, P-P*
CO/H2
+
CHO
CHO
+
NR
NR
NR
NR
O
O
O
4
5β
5α
6
4a R = H
4b R = BOC
4c R = CH2COOCH2-Ph
OTBS
O
COOH
NR
3
Scheme 2.
vent (benzene, 30mL), pressure (60atm), temperature
(60°C), CO/H2 ratio (1:1) and substrate concentration
(0.017M) were kept constant. The Rh(I) concentration
was adjusted to obtain the proper substrate/catalyst
ratio; withall our ligands we always obtained homoge-
neous solutions; no precipitation of the catalyst was ob-
served. When the aminophosphonite–phosphinites 7
were used, no appreciable differences were seen when
the chirality of the binaphthol changed from S to R
(entries 1 and 2) and when the chirality of the backbone
changed from S to R (entries 2 and 3); the branched
aldehydes prevailed but the diastereoselectivity was dis-
appointingly low, to predominantly give the undesired
5a isomer. The only difference was that 7-(SS) gives a
more efficient catalyst witha catalyst activity (TOF) of
at least 15 times higher than the other ligands of the
same type (entry 2). Analogous results were obtained
when aminophosphine–phosphites 8 were used (entries
4, 5 and 6); also in this case the (S,S) stereochemistry
of the ligand gave rise to a very efficient catalyst (entry
6). When the substrate/catalyst ratio was increased to
1000/1, the diastereoselectivity remained unchanged,
although the regioselectivity and catalyst activity were
strongly reduced. These results indicate that amino-
phosphonite–phosphinites and aminophosphine–phos-
phites closely resemble the behaviour shown by (R)-
and (S)-Binap.6 Contrary to all expectations we found
that also the regio- and diastereoselectivity of (S,S)-
BDPP-(NBD)Rh+B(Ph)4 are strongly dependent on
the substrate/catalyst ratio; in fact changing the ratio
from a 20/1 (5mol%) to 1000/1 meant that the regiose-
lectivity of the (S,S)-BDPP-(NBD)Rh+B(Ph)4 catalyst
dropped from 97/3 to 61/39 (branched/linear) while the
diastereoselectivity was dramatically reversed, changing
from 91/9 to 41/59 (5b/5a) (entries 8, 9 and 10).
O
O
*
O
P(C6H5)2
O
P
*
N
N
P
P
O
O
7
8
7 = (SS, RR, SR, RS)
Aminophosphonite-phosphinite
8 = (SS, RR, SR, RS)
Aminophosphine-phosphite
Scheme 3.
atom on the aminoalcohol backbone with the stereo-
genic axis of the 1,10-binaphthyl moiety.
The Rh(acac)(CO)2 complexes withthese ligands gave
very high regioselectivities and satisfactory enantioselec-
tivities in the asymmetric hydroformylation of vinylace-
tate.8 The stereodifferentiating ability of the catalyst
derived from these ligands and the structural similarities
to binaphthols or to the other bisphosphites, which seem
to be the ligands of choice for the asymmetric hydro-
formylation,9 prompted us to investigate aminophos-
phonite–phosphinite and aminophosphine–phosphite
ligands in the preparation of 1b-methylcarbapenem by
the asymmetric hydroformylation of different N-substi-
tuted azetidinones, 4a–c.
2. Results and discussion
The asymmetric hydroformylation of enantiomerically
pure azetidinones 4a–c are summarized in Table 1.
Due to the fact that in our preliminary investigations8
we did not find any remarkably different activities
between Rh(acac)(CO)2 and the zwitterionic (NBD)-
Rh+B(Ph)4 complex, we concentrated our efforts only
on the catalytic system generated Ôin situÕ by mixing
the zwitterionic Rh(I) complex with the appropriate lig-
and in a 1:2 ratio. The reaction conditions, that is, sol-
The introduction of the bulky electron deficient t-Boc
group reduces dramatically the regioselectivity in the
hydroformylation of 4b; linear aldehyde 6b became pre-
dominant whatever ligand was used; this behaviour clo-
sely resembles that of (S,S)-BDPP-(NBD)Rh+B(Ph)4
(entries 11–18 and entry 19). The presence of the t-Boc

E. Cesarotti, I. Rimoldi / Tetrahedron: Asymmetry 15 (2004) 3841–3845
Table 1. Asymmetric hydroformylation of 4a–c
3843
Entry
Substrate
Catalyst
Sub/cat
Yield (%)
Time (h)
5a+5b/6
5b/5a
1
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4b
4b
4b
4b
4c
4c
4c
4c
4c
7-RR/Rh(NBD)B(Ph)4
7-SS/Rh(NBD)B(Ph)4
7-RS/Rh(NBD)B(Ph)4
8-SR/Rh(NBD)B(Ph)4
8-RR/Rh(NBD)B(Ph)4
8-SS/Rh(NBD)B(Ph)4
8-SS/Rh(NBD)B(Ph)4
(SS)BDPP/Rh(NBD)B(Ph)4
(SS)BDPP/Rh(NBD)B(Ph)4
(SS)BDPP/Rh(NBD)B(Ph)4
7-RR/Rh(NBD)B(Ph)4
7-RS/Rh(NBD)B(Ph)4
7-SS/Rh(NBD)B(Ph)4
7-SR/Rh(NBD)B(Ph)4
8-RR/Rh(NBD)B(Ph)4
8-SS/Rh(NBD)B(Ph)4
8-SR/Rh(NBD)B(Ph)4
8-SS/Rh(NBD)B(Ph)4
(SS)BDPP/Rh(NBD)B(Ph)4
7-SR/Rh(NBD)B(Ph)4
7-RS/Rh(NBD)B(Ph)4
8-SR/Rh(NBD)B(Ph)4
8-RS/Rh(NBD)B(Ph)4
(SS)BDPP/Rh(NBD)B(Ph)4
20/1
20/1
20/1
20/1
20/1
100/1
1000/1
20/1
80/1
1000/1
20/1
20/1
20/1
20/1
20/1
20/1
20/1
500/1
20/1
20/1
20/1
20/1
20/1
20/1
100
93
60
4
68/32
62/38
64/36
69/31
58/42
63/37
52/48
97/3
39/61
40/60
40/60
40/60
39/61
38/62
39/61
91/9
2
3
100
100
100
100
94
60
64
17
17
167
24
88
114
17
60
78
78
17
17
40
168
24
90
90
48
168
48
4
5
6
7
8
100
90
9
81/19
61/39
52/48
26/74
48/52
12/88
24/76
44/56
27/73
7/93
37/63
41/59
38/62
40/60
66/34
61/39
78/22
75/25
78/22
n.d.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
93
100
100
100
100
95
100
100
50
100
100
100
100
100
0
13/87
30/70
1/>99
14/86
21/79
—
>99/1
60/40
n.d.
62/38
60/40
—
Reactions were carried out under 60 atmos of a 1/1 mixture of CO/H2, at 60°C, witha ligand/catalyst ratio = 2/1 and a substrate concentration
0.017M. The % conversion and the ratio are determined by 1H NMR spectroscopy and by GC–MS.
group at nitrogen implies a great change in the diastereo-
selectivity; the aminophosphine–phosphite ligands 8 al-
ways gave the desired b-isomer withdiastereoselectivities
up to 78/22 (entries 15, 16 and 17). Withaminophospho-
nite–phosphinites 7, 7-(SS) and 7-(SR) gave the b-isomer
(entries 13 and 14) while the 7-(RR) and 7-(RS)
ligands gave the a-isomer (entries 11 and 12). When the
substrate/catalyst ratio was increased to 500/1, the only
product obtained was the linear aldehyde 6b (entry 18).
The introduction at the nitrogen of the less electron defi-
cient group –CH2COOCH2Ph implies that the regiose-
lectivity is reduced and the linear aldehyde 6c is always
the prevailing product; both aminophosphonite–phos-
phinite ligands 7 and aminophosphine–phosphite ligands
8 however, give the desired 5cb-isomer prevailing on the
5ca-isomer (entries 20–23). The chiralities of the ligands
seem to only affect the overall productivities of the cata-
lysts (entry 23 compared to entries 20–22). It is notewor-
thy that (S,S)-BDPP-(NBD)Rh+B(Ph)4 is completely
inactive on substrate 4c (entry 22).
nophosphonite–phosphinite–Rh(I) complexes seems to
be independent from the substrate/catalyst ratio.
4. Experimental
The aminophosphonite–phosphite and aminophosphon-
ite–phosphinite ligands and catalysts were prepared
according to the literature procedure,8 under an inert
atmosphere (argon) using standard Schlenk techniques.
Catalytic reactions were performed in a 200mL stainless
steel autoclave equipped withtemperature control and
magnetic stirrer. Unless otherwise stated, the other
materials were obtained from commercial suppliers
and used without further purification. The rhodium
zwitterionic catalyst, (NBD)Rh+B(Ph)4, was prepared
according to the literature procedure10 as well as the
4-vinyl b-lactam 5a.11
1H NMR spectra are recorded on a Bruker AC300
equipped witha non-reverse probe and also or on a Bru-
ker DRX300 Avance. GC–MS spectra are recorded on
Thermo Finningan MD 800 equipped with GC Trace
(SE 52 column: length25m, / int. 0.32mm, film 0.4–
0.45lm).
3. Conclusions
Aminophosphonite–phosphite and aminophosphonite–
phosphinite–Rh(I) complexes catalyze the asymmetric
hydroformylation of 4-vinyl azetidin-2-one to (3S,4R)-
4-[(R)-10-formylethyl]azetidin-2-one, a pivotal interme-
diate for the synthesis of 1b-methylcarbapenem. A bulky
group must be present on the nitrogen to drive the reac-
tion withgood diastereoselectivity towards the derived b-
isomer, even if this implies a reduction in the regioselec-
tivity to the branched aldehyde. Unlike the behaviour of
other active and successful Rh(I) complexes, the diaste-
reoselectivity of aminophosphonite–phosphite and ami-
4.1. Preparation of 4b
Triethylamine (140lL, 1.1mmol) was added to a mix-
ture of di-(tert-butyl)dicarbonate (436mg, 1.1mmol),
4a (255mg, 1mmol) and DMAP (123.4mg, 1.1mmol)
in methyl chloride (10mL). The reaction mixture was
stirred for 6h at room temperature and then quenched
withsaturated NH Cl and extracted withethyl acetate
4
(20mL). The combined organic layers were dried over

3844
E. Cesarotti, I. Rimoldi / Tetrahedron: Asymmetry 15 (2004) 3841–3845
anhydrous Na2SO4 and concentrated in vacuo. Purifica-
tion of the residue by flash chromatography on silica gel,
with hexane/ether (1:10) as eluants, gave 302mg (85%)
2.71–2.80 (dd, 1H), 3.59–3.63 (d, 2H), 4.10–4.20 (m,
2H), 6.1 (br, 1H), 9.77 (s, 1H); C14H27NO3Si calcd
285.18, found 228.2 (M+ꢀ57, –C4H9).
1
of 4b as a yellow oil. H NMR: d 0.054 (s, 3H), 0.061
1
(s, 3H), 0.85 (s, 9H), 1.18–1.21 (dd, 3H), 1.48 (s, 9H),
2.86–2.90 (m, 1H), 4.27–4.32 (m, 1H), 4.50–4.54 (m,
1H), 5.24–5.44 (dd, 2H), 5.88–6.00 (m, 1H); C18H33NO4-
Si calcd 355.55, found 298.2 (M+ꢀ57, –C4H9); IR
(CH2Cl2): 1722.59 (C@O, lactam), 1805.53 (C@O,
Compound 5ba: H NMR: d 0.02 (s, 3H), 0.03 (s, 3H),
0.85 (s, 6H), 1.12–1.15 (d, 6H), 1.49 (s, 9H) 2.73–2.87
(m, 1H), 3.98–4.32 (m, 1H), 4.33–4.48 (m, 1H), 5.26–
5.45 (m, 2H), 5.96–6.00 (m, 1H), 6.8 (br, 1H), 9.69 (s,
1H); C19H35NO5Si calcd 385.58, found 328.5 (M+ꢀ57,
–C4H9).
carbamate)cmꢀ1
.
1
4.2. Preparation of 4c
Compound 5bb: H NMR: d 0.02 (s, 3H), 0.03 (s, 3H),
0.85 (s, 6H), 1.12–1.15 (d, 6H), 1.49 (s, 9H) 2.73–2.87
(m, 1H), 3.98–4.32 (m, 1H), 4.33–4.48 (m, 1H), 5.26–
5.45 (m, 2H), 5.96–6.00 (m, 1H), 6.8 (br, 1H), 9.69 (s,
1H); C19H35NO5Si calcd 385.58, found 328.5 (M+ꢀ57,
–C4H9).
Compound 4a (510mg, 2mmol) in THF (3mL) was
added to a solution of NaH (55mg, 2.2mmol) in THF
(10mL) at 0°C. The reaction mixture was stirred for
30min, and then benzyl bromoacetate (458mg, 2mmol)
in THF (3mL) added at ꢀ78°C. The reaction mixture
was stirred for 6h while the temperature of the reaction
was allowed to run to room temperature, then treated
1
Compound 6b: H NMR: d 0.03 (s, 3H), 0.059 (s, 3H),
0.87 (s, 6H), 1.13–1.17 (d, 6H), 1.54 (s, 9H) 2.73–2.87
(m, 1H), 3.86–4.25 (m, 1H), 4.35–4.49 (m, 1H), 5.31–
5.47 (m, 2H), 5.93–5.99 (m, 1H), 6.5 (br, 1H), 9.73 (s,
1H); C19H35NO5Si calcd 385.58, found 328.5 (M+ꢀ57,
–C4H9).
withsaturated NH Cl and the aqueous solution further
4
extracted with ether (40mL) and then with methylene
chloride (20mL). The combined organic layers were
dried over anhydrous Na2SO4 and concentrated in
vacuo. Purification of the residue by flash chromatogra-
phy on silica gel, with hexane/ether (1:9) as eluants, gave
334.5mg (83%) of 5c as a yellow oil. 1H NMR: d 0.025 (s,
3H), 0.059 (s, 3H), 0.87 (s, 9H), 1.22–1.24 (d, 3H), 2.91–
2.94 (dd, 1H), 3.81–3.86 (m, 2H), 4.05–4.22 (m, 2H),
4.68–4.70 (m, 2H) 5.10–5.33 (m, 2H), 5.82–5.84 (m,
1H), 7.20–7.37 (m, 5H); C22H33NO4Si calcd 403.59,
found 346.3 (M+ꢀ57, –C4H9); IR (CH2Cl2): 1730.70
1
Compound 5ca: H NMR: d 0.01 (s, 3H), 0.02 (s, 3H),
0.9 (s, 9H), 1.20–1.22 (d, 6H), 1.70–1.82 (m, 1H), 2.0–
2.2 (m, 1H), 2.4–2.65 (m, 2H), 2.85–2.95 (m, 1H),
3.76–4.2 (m, 2H), 5.1–5.3 (m, 1H), 7.3–7.6 (m, 5H),
9.73 (s, 1H); C23H35NO5Si calcd 433.62, found 376.2
(M+ꢀ57, –C4H9).
1
(C@O, lactam), 1825.42 (C@O, carbamate)cmꢀ1
.
Compound 5cb: H NMR: d 0.01 (s, 3H), 0.02 (s, 3H),
0.9 (s, 9H), 1.20–1.22 (d, 6H), 1.70–1.82 (m, 1H), 2.0–
2.2 (m, 1H), 2.4–2.65 (m, 2H), 2.85–2.95 (m, 1H),
3.76–4.2 (m, 2H), 5.1–5.3 (m, 1H), 7.3–7.6 (m, 5H),
9.73 (s, 1H); C23H35NO5Si calcd 433.62, found 376.2
(M+ꢀ57, –C4H9).
Compound 6c: 1H NMR: d 0.01 (s, 3H), 0.02 (s, 3H), 0.9
(s, 9H), 1.20–1.22 (d, 6H), 1.70–1.82 (m, 1H), 2.0–2.2
(m, 1H), 2.4–2.65 (m, 2H), 2.70–2.72 (m, 1H), 3.76–4.2
(m, 2H), 5.1–5.3 (m, 1H), 7.3–7.6 (m, 5H), 9.73 (s,
1H); C23H35NO5Si calcd 433.62, found 376.2 (M+ꢀ57,
–C4H9).
4.3. General procedure for hydroformylation of 4
In a typical run, the 4-vinyl b-lactam 4 (0.5mmol), the
(NBD)Rh+B(Ph)4 complex (5mmol% to the substrate),
and phosphorus ligand (10mmol% to the substrate)
were placed in a Schlenk tube, at which point benzene
(30mL) was added, the resulting solution stirred for
20min and then transferred to a stainless steel autoclave
previously purged five times witha H 2/CO mixture. The
autoclave was pressured and heated in an oil bath. At
the end of the reaction, the autoclave was vented and
the solvent distilled. The branched/linear ratio 5/6, th e
conversion and the diastereomeric excess of 5 were
Acknowledgements
1
determined by H NMR and GC–MS.
`
This work was supported by the FIRST, Universita
1
Compound 5aa: H NMR: d 0.01 (s, 3H), 0.03 (s, 3H),
degli Studi di Milano. We are grateful to A.C.S. Dobfar
s.p.a. for the generous gift of the starting material, 4-
acetoxy-2-azetidinone.
0.81 (s, 6H), 1.21–1.27 (d, 6H), 2.45–2.63 (m, 1H),
2.73–2.85 (dd, 1H), 3.60–3.62 (d, 2H), 4.10–4.17 (m,
2H), 6.2 (br, 1H), 9.68 (s, 1H); C14H27NO3Si calcd
285.18, found 228.2 (M+ꢀ57, –C4H9).
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Compound 5ab: H NMR: d 0.01 (s, 3H), 0.02 (s, 3H),
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1
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