Baker's yeast reduction of cyclic δ-ketoesters: Synthesis and chiroptical properties of condensed δ-lactones

Article abstract of DOI:10.1016/S0957-4166(99)00098-1

Enantiomerically pure condensed δ-lactones have been prepared from the corresponding δ-ketoesters by the use of Saccharomyces cerevisiae. The reactions were not only highly enantioselective but also highly diastereoselective, provided the baker's yeast was preincubated at 50°C for 30 min. Interestingly, and contrary to what is usually found, the use of nutrients inhibited the bioreductions. The relative configurational assignments have been made by means of NMR, while the absolute configurations and conformations of the lactone rings were attributed by means of CD studies.

Full text of DOI:10.1016/S0957-4166(99)00098-1

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1243–1254
Baker’s yeast reduction of cyclic δ-ketoesters: synthesis and
chiroptical properties of condensed δ-lactones
Cristina Forzato, Patrizia Nitti, Giuliana Pitacco and Ennio Valentin ∗
Dipartimento di Scienze Chimiche, Università di Trieste, via Licio Giorgieri 1, I-34127 Trieste, Italy
Received 29 January 1999; accepted 11 March 1999
Abstract
Enantiomerically pure condensed δ-lactones have been prepared from the corresponding δ-ketoesters by the use
of Saccharomyces cerevisiae. The reactions were not only highly enantioselective but also highly diastereoselective,
provided the baker’s yeast was preincubated at 50°C for 30 min. Interestingly, and contrary to what is usually found,
the use of nutrients inhibited the bioreductions. The relative configurational assignments have been made by means
of NMR, while the absolute configurations and conformations of the lactone rings were attributed by means of CD
studies. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
The δ-lactone ring is present in a variety of natural products1 isolated from insects, plants, fungi
and marine organisms, normally as part of complex polycyclic skeletons. Examples can be found in
quassinoid derivatives such as sergeolide2 (isolated from Picrolemma pseudocoffea) and simalikalactone
D3 (isolated from the wood of Simaba multiflora) as well as in diterpenoids such as epinodosinol4
(isolated from Rabdosia angustifolia) and longirabdolide G5 (isolated from Rabdosia longituba). Many
of them show biological activity; for instance, simalikalactone D is an antitumour while sergeolide is an
antimalarial.
Therefore, a strategy giving access to optically active δ-lactones as chiral building blocks for the
synthesis of more complex compounds seems highly interesting.
2. Results and discussion
In our previous papers6,7 enantiomerically pure bicyclic γ-lactones were synthesized from the ap-
propriate γ-nitroketones and γ-ketoesters, using raw baker’s yeast in the enantiodifferentiating step.
∗
Corresponding author. Tel: +39-040-6763917; fax: +39-040-6763903; e-mail: valentin@dsch.univ.trieste.it
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00098-1

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Since these bioreductions were not only highly enantioselective but also highly diastereoselective, it
seemed interesting to verify whether the same strategy could also be useful for the synthesis of bicyclic
δ-lactones, in particular starting from δ-ketoesters.
2.1. Synthesis of the substrates
Substrates used were the δ-ketoesters 3, 4 and 5 (Scheme 1), derived from a Michael-type reaction
between ethyl acrylate and the corresponding pyrrolidino cyclohexenes 1 and 2, followed by hydrolysis
under equilibrating acidic conditions.8 The δ-ketoesters 4 and 5 could not be separated by column
chromatography and, therefore, a mixture of thermodynamic formation (4:5, 9:1) was used in the
subsequent steps.
Scheme 1.
2.2. Chemical reduction
Compound 3 and the mixture of 4 and 5 were reduced with both a chemical agent and baker’s yeast.
As reported in the literature,9 among the reducing agents for δ-ketoesters, sodium cyanoborohydride is a
better choice than sodium borohydride because it gives no by-products and also the diastereoselectivity
is higher. Reduction of δ-ketoester 3 gave a mixture of cis- and trans-fused lactones 610–12 and 710 in
the ratio 1:4, determined by HRGC. The δ-hydroxyester intermediates could not be detected, as they
immediately cyclized to the corresponding δ-lactones in the reaction medium.
Similarly, when the 9:1 mixture of 4 and 5 was reduced under the same conditions, three lactones were
identified, namely the cis-fused lactone 89,13 (18%), the trans-fused lactone 9 (72%) both derived from
4, and the cis-fused lactone 10 (10%) derived from 5.
1
The geometry of the δ-lactones was established by analyzing their H NMR spectra. In particular,
in the cis-fused lactones 6 and 8 the respective oxygen atom was essentially axial, as indicated by the
chemical shift and pattern of its geminal protons at C-8a (4.49 ppm, pseudo q, WH 7.2 Hz and 4.48
ppm, pseudo q, WH 5.2 Hz, respectively). In 7 and 9 the same protons H-8a were clearly axial as can be
inferred from the values of the vicinal coupling constants as well as from the values of their respective
chemical shifts (3.88 ppm, dt, J1=J2 10.4 Hz, J3 4.4 Hz and 3.84 ppm, dt, J1=J2 10.5 Hz, J3 4.4 Hz,
respectively).

C. Forzato et al. / Tetrahedron: Asymmetry 10 (1999) 1243–1254
1245
2.3. Baker’s yeast reduction
Having isolated and characterized all the lactones formed by chemical reduction of the δ-ketoesters,
the same substrates were reduced with baker’s yeast in water. The conditions used for bioreduction did
not allow for the isolation of the δ-hydroxyester intermediates and the corresponding δ-lactones were
directly formed.
A few parameters were checked on ketoester 3 to optimize the reaction conditions, namely (a) use
of raw14 and dry yeast,15 (b) preincubation of yeast,16 (c) presence of glucose,17 and (d) presence of
ethanol.18 The results are summarized in Table 1.
First, as anticipated, the presence of either glucose17 as a nutrient or ethanol,18 which is usually
considered an inhibitor but in some cases was found to be an energy source,18 greatly inhibited the
bioreduction (in 18 days a conversion of only 8% was detected by HRGC). This was independent of the
type of baker’s yeast used. This result is quite unusual because this is the first time, as far as we know,
that a nutrient (glucose) has acted as an inhibitor.
Pretreatment of yeast seemed to be an important factor for raw yeast, whereas it plays no role for dry
yeast. In fact, preincubation of raw yeast at 50°C for 30 min increased the diastereoselectivity of the
reduction (entry 2, Table 1), with the enantioselectivity remaining high. According to Nakamura et al.,16
heat treatment denatures thermolabile dehydrogenases, thus increasing diastereoselectivity. It is evident,
therefore, that these particular dehydrogenases are not present in dry yeast.
Actually, when the ketoester 3 was reduced with preincubated raw yeast, under a N2 atmosphere,† the
corresponding cis-fused lactone (−)-6 was obtained in 98% diastereomeric as well as enantiomeric excess
(entry 2). It is interesting to note, however, that reductions with dry baker’s yeast were also satisfactory,
provided the yeast was preincubated, as they furnished the lactone (−)-6 with 92–94% d.e. and 99% e.e.
Conversions ranged from 36% to 78%, the values exceeding 50% being due to the equilibration between
the enantiomers of 3 occurring during the reaction.
Table 1
Reduction of the δ-ketoester (ꢀ)-3 with baker’s yeast
Therefore, reaction conditions, which gave the best result for the bioreduction of compound 3, were
used for reducing the thermodynamic mixture of δ-ketoesters 4 and 5. This latter reaction was not only
†
Since these bioreductions last more than 10 days, a nitrogen atmosphere is necessary to prevent yeast from rotting away; of
course these can be done only if no nutrients are used.

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highly enantioselective (99% e.e.) but also highly diastereoselective, leading to the lactone (−)-8 as a
single stereoisomer, after a 50% conversion. Therefore, of all the lactones shown in Scheme 1, it is
possible to selectively obtain cis-fused lactones (−)-6 and (−)-8 directly and in high enantiomeric excess
using preincubated raw baker’s yeast.
From the bioreduction of the 9:1 mixture of 4 and 5, the unreacted ketoester (−)-4, having 96% e.e.,
was recovered after chromatographic separation. It was reduced with sodium cyanoborohydride thus
allowing for the isolation of the trans-fused lactone (−)-9 in high enantiomeric excess. In fact, the reaction
furnished a 4:1 mixture of the cis- and trans-fused lactones (+)-8 and (−)-9, respectively, which were
separated by flash chromatography (Scheme 2).
Scheme 2.
2.4. Determination of the absolute configuration of compounds (−)-4, (−)-6, (−)-8 and (−)-9
Absolute configuration of the enantiomerically pure compounds was established by means of CD
spectroscopy. The CD spectrum of the unreacted ketoester (−)-4 showed a positive Cotton effect at 289
nm ([Θ]=+2248) for the n -→ π* transition of the ketone carbonyl group. Using the simple octant rule19
none of the substituents would make any contribution to the Cotton effect as both lie on symmetry
planes. However, a large positive CE was observed for (+)-(2S,4S)-2-methyl-4-t-butylcyclohexanone
and molecular mechanics calculations showed that the chair conformation is favoured.19a Geometry
optimization up to a gradient of 10−4 kcal/molÅ performed with PM320 Hamiltonian afforded four
possible conformations with enthalpy differences of almost 0.90 kcal/mol. In the present case the methyl
group is substituted for a slightly longer chain that might jut into one of the front octant regions with
concomitant large signed contribution. In any case, the large positive CE is consistent with the 2S,4S-
configuration of the stereogenic centres of (−)-4. On the other hand, similar spectra were obtained
for 2-(2-nitroethyl)-4-t-butylcyclohexanone7a having the same absolute configuration as (−)-4. As a
consequence, absolute configurations of lactones (+)-8 and (−)-9, both derived from (−)-4, are 4aS,6S,8aR
and 4aS,6S,8aS, respectively, as shown in Scheme 2.
The absolute configuration of the enantiomeric lactone (−)-8, derived from the reduction of δ-ketoester
(ꢀ)-4 with baker’s yeast, is 4aR,6R,8aS. The carbinol carbon atom in the hydroxyester intermediate,
which was not isolated, was therefore S. Thus, once again the enantiopreference shown by the yeast is in
accordance with Prelog’s rule.21 The cis-fused lactone (−)-8 exhibited a negative Cotton effect at 212 nm
as did (−)-6 (Fig. 1). Therefore, C-4a and C-8a in (−)-6 can be attributed the same absolute configuration
as the same stereogenic centres in (−)-8, namely R and S, respectively.
An evaluation of the conformation of the lactone ring can be made from the CD curves.22 In accordance
with Korver,22c the boat conformation is responsible for a maximum appearing below 225 nm, while a
maximum above 225 nm is indicative of a half-chair conformation for the lactone ring. Therefore, a boat
conformation can be envisaged for both cis-fused δ-lactones (−)-6 and (−)-8 (Fig. 1 and Table 2), while
for the trans-fused system (−)-9 part of the half-chair conformation is surely present (Table 2).
In order to investigate the influence of steric factors on the equilibrium between chair and boat
conformers, the cis-fused lactones (−)-6 and (−)-8 were α-methylated under the conditions used by

C. Forzato et al. / Tetrahedron: Asymmetry 10 (1999) 1243–1254
1247
Figure 1. CD spectra of (−)-6, (−)-8, (−)-12 and (−)-14
Table 2
Solvent effect
Grieco23 for the stereoselective α-methylation of γ-lactones. In neither case was the stereoselectivity
complete. Methylation of (−)-6 afforded an 88:12 mixture of diastereoisomeric lactones (−)-11 and (−)-
12. Only lactone (−)-11 was obtained as a pure compound, although contaminated by 3% of (−)-12
(Scheme 3).
Analogous results were obtained for the methylation of (−)-9, which furnished (−)-13 and (−)-14,
besides the α-dimethylated lactone (−)-15. The ratio of (−)-13, (−)-14 and (−)-15 was 75:5:20. Separation
of the mixture afforded (−)-13 contaminated by 6% of its diastereoisomer (−)-14, while (−)-15 was
isolated as a pure compound.
It is interesting to underline that, although methylation of δ-lactones was not completely stereoselec-

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Scheme 3.
tive, as is usual for γ-lactones, the preferred approach of methyl iodide to the enolate intermediate is still
from the same side as that containing H-4a.
Inversion of configuration at C-3 of (−)-11 and (−)-13 furnished the corresponding isomers (−)-12
and (−)-14. Inversion of configuration was accomplished by protonation of the corresponding enolates.
Neither protonation was completely stereoselective. However, while (−)-12 was obtained in admixture
with (−)-11 (13%), (−)-14 could be isolated as a pure compound.
The 1H NMR data for 11–14 of H-8a (11: 4.50 ppm, pseudo q, WH 8.8 Hz; 12: 4.50 ppm, pseudo q,
WH 11.3 Hz; 13: 4.32 ppm, bq, WH 8.9 Hz; 14: 4.44 ppm, bq, WH 8.0 Hz) can be compared with the
values found for their non-methylated analogues 6 and 8, whose data are reported above. From these data
it can also be deduced that in the α-methylated systems the oxygen atom is essentially axial.
Orientation of the methyl group was established by means of DIFNOE measurements carried out on
lactone (−)-12 (Fig. 2). Of particular interest is the NOE effect found for H-3 when irradiating H-8a. It
is clearly consistent with the boat conformation of the lactone ring which is confirmed by the position
of the maximum in its CD curve (216 nm).22c Because of the boat conformation, the methyl group
bisects the H–C4–H angle, as proved by the same value of enhancement found for both H-4 (4%), when
irradiating the methyl group. The Cotton effect exhibited by (−)-12 is negative as it is for the analogous t-
butylated derivative (−)-14 (Fig. 2). Since (−)-12 derives from (−)-6, the absolute configuration of (−)-12
is 3R,4aR,8aS and that of (−)-14 is 3R,4aR,6R,8aS.
Figure 2. DIFNOE measurements on lactone (−)-12
3. Conclusions
All the configurational and conformational assignments made for the δ-lactones are in accordance
with the Legrand and Bucourt rule22e that states that a negative Cotton effect is expected for a positive
O–CO–Cα–Cβ torsional angle (Fig. 2). A negative Cotton effect was actually found for (−)-12 as well
as for (−)-6, (−)-8 and (−)-14.
The trans-fused lactone (−)-9 and the α-methylated lactones (−)-11 and (−)-13 exhibited bisignated
CEs with a positive maximum around 235 nm, and a negative one around 210 nm (Fig. 3). On the basis of
the previous considerations, an equilibrium between the boat and half-chair conformers can be envisaged
for the lactone ring. Furthermore, since the band at around 210 nm shows a negative CE, the absolute
configurations of (−)-9, (−)-11 and (−)-13 are 4aS,6S,8aS, 3S,4aR,8aS and 3S,4aR,6R,8aS, respectively.

C. Forzato et al. / Tetrahedron: Asymmetry 10 (1999) 1243–1254
1249
Figure 3. CD spectra of (−)-11 and (−)-13
Figure 4. Solvent dependency of the CD spectra of (−)-11
The existence of this conformational equilibrium for the lactone ring is also confirmed by the solvent
dependency of the CD spectra found for all lactones (Fig. 4 and Table 2).
4. Experimental
4.1. General
Melting points were determined with a Büchi apparatus and are uncorrected. IR spectra were recorded
in CHCl3, unless otherwise stated, on a Jasco FT/IR 200 spectrophotometer. 1H NMR spectra were run on
a Jeol EX-400 (400 MHz) spectrometer, using deuteriochloroform as the solvent and tetramethylsilane as
the internal standard. 13C NMR spectra were recorded on a Jeol EX-400 (100.5 MHz) instrument. Optical

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C. Forzato et al. / Tetrahedron: Asymmetry 10 (1999) 1243–1254
rotations were determined on a Perkin–Elmer Model 241 polarimeter. CD spectra were obtained on a
Jasco J-700A spectropolarimeter (0.1 cm cell); UV spectra were recorded on a Perkin–Elmer Lambda
2 and a Jasco V-550 spectrophotometers (1.0 cm cell) in methanol; GLC analyses were obtained on a
Carlo Erba GC 8000 instrument, the capillary column being carbowax (30 m×0.32 mm; carrier gas He,
40 kPa) and a Chiraldex™ type G-TA, trifluoroacetyl γ-cyclodextrin (40 m×0.25 mm; carrier gas He,
180 kPa) or DMePe β-cyclodextrin (25 m×0.26 mm; carrier gas He, 110 kPa). Mass spectra were run
by the electron-impact mode on a VG 7070 (70 eV) at the Central Facility for Mass Spectrometry of the
University of Trieste and on a Hewlett–Packard 5971-A GC–MS instrument. TLCs were performed on
Whatman K6F silica gel plates (eluant: light petroleum/ethyl acetate). Flash chromatography was run on
silica gel 230–400 mesh ASTM (Kieselgel 60, Merck). Light petroleum refers to the fraction with bp
40–70°C and ether to diethyl ether.
4.2. Synthesis of the substrates
Ethyl 3-(2-oxocyclohexyl)propionate 3 was prepared according to the literature.8 Its IR, 1H NMR and
13C NMR were identical with the reported values.24
Ethyl 3-(5-(1,1-dimethylethyl)-2-oxo-cyclohexyl)propionate 4 was prepared by treatment of 1-N-
pyrrolidinyl-4-(1,1-dimethylethyl)cyclohexene with ethyl acrylate in refluxing dioxane for 3 h. After
addition of water and refluxing for 1 h, extraction with ether afforded a mixture of 4 and 5 in a 9:1
ratio, which was the thermodynamic composition. The two isomers could not be separated by flash
chromatography. The spectroscopic data of compound (−)-4 recovered after the bioreduction are given.
4.2.1. (−)-cis-Ethyl 3-(5-(1,1-dimethylethyl)-2-oxo-cyclohexyl)propionate 4 (in admixture with 5% of
the trans-isomer 5)
IR (film): 1730, 1710 cm−1 (C_O); 1H NMR, δ, ppm: 4.12 (2H, q, J 7.3, COOCH2CH3), 2.36 (5H,
m), 2.09 (3H, m), 1.52 (3H, m), 1.25 (3H, t, J 7.3, COOCH2CH3), 1.20 (1H, m), 0.91 [9H, s, C(CH3)3];
13C NMR, δ, ppm: 212.7 (s), 173.6 (s), 60.2 (t), 48.7 (d), 47.0 (d), 41.6 (t), 35.3 (t), 32.4 (s), 31.9 (t),
28.8 (t), 27.6 (3q), 24.8 (t), 14.2 (q); MS, m/z: 254 (M+·, 13), 209 (37), 208 (51), 193 (17), 180 (14),
167 (10), 151 (25), 124 (51), 96 (22), 83 (25), 69 (17), 57 (100), 55 (59); 96% e.e. (by chiral HRGC);
[α]25=−18.1 (c 0.11, CH3OH); [Θ]289=+2248 (CH3OH); UV (CH3OH), λmax (ε, M−1 cm−1): 282 (28),
D
229 (145).
4.3. Chemical reduction of δ-ketoesters 3 and 4,5
Reduction of 3 with NaCNBH3 gave the corresponding lactones 6 and 7 in a 1:4 ratio. They were
separated by flash chromatography.
Reduction of the 9:1 mixture of 4 and 5 with NaCNBH39 afforded the corresponding lactones 8 and 9
in a 1:4 ratio. Lactone 10, derived from the reductive cyclization of 5, was also formed in 10%. Lactones
8, 9 and 10 were partially separated by flash chromatography affording 9 as a pure compound together
with a mixture of 8 and 10.
4.3.1. cis-6-(1,1-Dimethylethyl)-octahydro-2H-1-benzopyran-2-one 10
1
Only a few signals could be identified in the mixture: H NMR, δ, ppm: 4.45 (1H, dt, J1=J2 5.5, J3
11.3, H-8a), 2.65 (1H, ddd, J1 1.5, J2 5.9, J3 18.1, H-3), 2.47 (1H, dd, J1 7.3, J2 18.1 H-3), 2.30 (1H, m),
1.42 (1H, dt, J1=J2 13.2, J3 4.9), 0.85 [9H, s, C(CH3)3], addition of a small amount of C6D6 shifted the
H-8a signals of 10 at 4.45 from that of 8, thus allowing the identification of the absorption; 13C NMR,

C. Forzato et al. / Tetrahedron: Asymmetry 10 (1999) 1243–1254
1251
δ, ppm: 170.6 (s), 81.0 (d), 40.7 (d), 32.7 (d), 32.0 [s, C(CH3)3], 30.4 (t), 30.1 (t), 29.1 (t), 27.4 [3q,
(CH3)3], 25.4 (t), 20.8 (t).
4.4. Reduction with baker’s yeast
4.4.1. Raw baker’s yeast
Method A: 100 g of raw baker’s yeast in 200 ml of water was added to 10 mmol of the δ-ketoester and
the mixture was stirred at room temperature.
Method B: 100 g of raw baker’s yeast in 200 ml of water was preincubated for 30 min at 50°C, added
to 10 mmol of the δ-ketoester and the mixture was stirred at room temperature.
4.4.2. Dry baker’s yeast
An amount of 4 g/mmol dry baker’s yeast purchased from Sigma–Aldrich was used following the
above procedures for methods A and B.
The course of the reduction was checked every 2 days by HRGC. At the end of the reaction, brine was
added and the broth was continuously extracted with diethyl ether for 48 h. The organic phase was dried
and evaporated.
4.4.3. (−)-(4aS,8aS)-cis-Octahydro-2H-1-benzopyran-2-one 6
1
IR, H NMR, 13C NMR were identical with the reported values.10a MS, m/z: 154 (M+·, 10), 110
(29), 98 (49), 82 (88), 81 (100), 69 (85), 68 (95), 55 (88); 98% e.e. (β-cyclodextrin), [α]25=−45.7 (c
D
0.35, CH3OH); [α]25=−53.5 (c 0.4, THF) [lit.11 [α]D=−28 (c 2, THF)]; [Θ]213=−4345 (CH3OH); UV
D
(CH3OH), λmax (ε, M−1 cm−1): 209 (87).
4.4.4. trans-Octahydro-2H-1-benzopyran-2-one 7
IR, 1H NMR, 13C NMR were identical with the reported values.10a
4.4.5. (−)-(4aR,6R,8aS)-cis-6-(1,1-Dimethylethyl)-octahydro-2H-1-benzopyran-2-one 8
Mp 99–100°C (from light petroleum); elemental analysis: calculated: C, 74.24; H, 10.54%; found: C,
74.00; H, 10.75%; IR (Nujol): 1740 (–O–C_O); 1H NMR, δ, ppm: 4.45 (1H, pseudo q, H-8a), 2.52 (2H,
dd, J1 6.6, J2 8.3, H-3), 2.14 (2H, m, H-4 and H-8), 1.88 (1H, m, H-6), 1.56 (4H, m, H-4, H-5, H-7,
H-8), 1.30 (1H, m, H-7), 1.23–1.08 (2H, m, H-5, H-6), 0.86 [9H, s, C(CH3)3]; 13C NMR, δ, ppm: 172.8
(s, C-2), 77.3 (d, C-8a), 47.2 (d, C-6), 33.2 (d, C-4a), 32.4 [s, C(CH3)3], 30.9 (t, C-8), 27.4 [3q, (CH3)3],
27.3 (t, C-5), 26.4 (t, C-3), 25.1 (t, C-4), 20.4 (t, C-7); MS, m/z: 195 (M−CH+3 , 5), 154 (83), 136 (42), 95
(37), 94 (100), 82 (38), 81 (32), 79 (21), 73 (18), 71 (20), 67 (25), 57 (53), 55 (35), 41 (56); >99% e.e.
(β-cyclodextrin); [α]25=−11.6 (c 0.25, CH3OH); [Θ]214=−6067 (CH3OH); UV (CH3OH), λmax (ε, M−1
D
cm−1): 212 (69).
4.4.6. (−)-(4aS,6S,8aS)-trans-6-(1,1-Dimethylethyl)-octahydro-2H-1-benzopyran-2-one 9
Oil; IR (film): 1740 (–O–C_O); 1H NMR, δ, ppm: 3.84 (1H, dt, J1=J2 10.5, J3 4.4, H-8a), 2.68 (1H,
m, H-3), 2.54 (1H, m, H-3), 2.15 (1H, m, H-8), 1.87 (3H, m, H-4, H-5, H-7), 1.50 (3H, m, H-4, H-4a,
H-8), 1.11 (2H, m, H-6, H-7), 0.87 [9H, s, (CH3)3], 0.86 (1H, m, H-5); 13C NMR, δ, ppm: 171.9 (s,
C-2), 83.6 (d, C-8a), 47.2 (d, C-6), 38.7 (d, C-4a), 32.4 [t, C-8 and s, C(CH3)3], 32.1 (t, C-5), 29.9 (t,
C-3), 27.7 [3q, (CH3)3], 26.7 (t, C-4), 25.0 (t, C-7); MS, m/z: 195 (M−CH+3 , 4), 167 (4), 155 (15), 136
(9), 94 (19), 57 (100), 41 (31); 96% e.e. (γ-cyclodextrin); [α]25=−33.5 (c 0.37, CH3OH); [Θ]235=+694,
D
[Θ]208=−1344, [Θ]191=−4480 (CH3OH); UV (CH3OH), λmax (ε, M−1 cm−1): 213 (90).

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4.5. α-Methylated lactones
4.5.1. (−)-(3S,4aS,8aS)-cis-3-Methyl-octahydro-2H-1-benzopyran-2-one 11 (in admixture with 3% of
12)
IR (Nujol): 1735 (–O–C_O); 1H NMR, δ, ppm: 4.50 (1H, pseudo q, WH 8.8, H-8a), 2.62 (1H, ddq,
WH 28.8, H-3), 2.01 (1H, m, H-8), 1.90 (2H, m, H-4, H-4a), 1.74 (1H, m, H-6), 1.68 (1H, ddd, H-4), 1.51
(5H, m, 2 H-5, 2 H-7, H-8), 1.30 (4H, d and m, CH3 and m); 13C NMR, δ, ppm: 175.1 (s, C-2), 79.2 (d,
C-8a), 34.2 (t, C-4), 33.3 (d, C-4a), 31.4 (d, C-3), 30.7 (t, C-8), 24.9 (t, C-5), 24.5 (t, C-6), 19.5 (t, C-7),
17.9 (q, CH3); MS, m/z: 124 (8), 109 (10), 98 (14), 95 (24), 82 (96), 81 (59), 67 (100), 56 (43); 98%
e.e.; [α]25=−30.0 (c 0.12, CH3OH); [Θ]233=+1474, [Θ]210=−884, [Θ]193=−8463 (CH3OH); UV, λmax (ε,
D
M−1 cm−1): 215 (106).
4.5.2. (−)-(3R,4aS,8aS)-cis-3-Methyl-octahydro-2H-1-benzopyran-2-one 12 [in admixture with 11
(13%)]
IR (Nujol): 1740 (–O–C_O); 1H NMR, δ, ppm: 4.50 (1H, pseudo q, WH 11.3, H-8a), 2.60 (1H, ddq,
H-3), 2.33 (1H, dt, J1=J2 8.7, J3 13.7, H-4), 2.00 (2H, m, H-4a and H-8), 1.64 (3H, m, H-5, H-7 and H-8),
1.51 (2H, m, H-6 and H-7), 1.31 (2H, m, H-5 and H-6), 1.19 (3H, d, J 6.7, CH3), 1.04 (1H, dt, J1=J2
13.7, J3 4.0, H-4); 13C NMR, δ, ppm: 176.6 (s, C-2), 75.8 (d, C-8a), 33.5 (t, C-4), 33.2 (d, C-4a), 32.5 (d,
C-3), 29.7 (2t, C-6, C-8), 24.6 (t, C-5), 20.3 (t, C-7), 15.8 (q, CH3); MS, m/z: 124 (7), 109 (10), 98 (12),
95 (22), 82 (89), 81 (55), 67 (100), 56 (51); 98% e.e.; [α]25=−67.7 (c 0.13, CH3OH); [Θ]216=−11114
D
(CH3OH); UV (CH3OH), λmax (ε, M−1 cm−1): 211 (107).
4.5.3. (−)-(3S,4aR,6R,8aS)-cis-6-(1,1-Dimethylethyl)-3-methyl-octahydro-2H-1-benzopyran-2-one 13
Mp 147°C (from light petroleum); elemental analysis: calculated: C, 74.95%; H, 10.78%; found: C,
1
74.7%; H, 10.9%; IR (Nujol): 1730 (–O–C_O); H NMR (CDCl3+10% C6D6), δ, ppm: 4.32 (1H, bq,
WH 8.9, H-8a), 2.51 (1H, ddq, J1 11.7, J2 7.3, J3 7.3, H-3), 2.06 (1H, dq, J1=J2=J3 2.9, J4 14.2, H-8),
1.73 (1H, ddd, J1 2.4, J2 7.3, J3 13.7, H-4), 1.66 (1H, m, WH 21.4, H-4a), 1.60 (1H, ddd, H-4), 1.54 (3H,
m), 1.53 (1H, m, H-7), 1.38 (2H, m, H-5+H-8), 1.24 (3H, d, J 7.3, CH3), 1.22 (1H, m, H-7), 1.12 (1H, m,
H-5), 1.02 (1H, m, H-6), 0.83 [9H, s, C(CH3)3]; 13C NMR (CDCl3+10% C6D6), δ, ppm: 175.0 (s, C-2),
78.6 (d, C-8a), 47.1 (d, C-6), 34.8 (t, C-4), 34.4 (d, C-4a), 32.4 [s, C(CH3)3], 31.3 (t+d, C-8+C-3), 27.3
[3q, (CH3)3], 25.8 (t, C-5), 20.3 (t, C-7), 17.7 (q, CH3); MS, m/z: 209 (M−CH+3 , 3), 168 (M−C4H8+·, 75),
123 (16), 95 (100), 87 (50), 82 (37), 81 (32), 69 (16), 67 (26), 57 (C4H+9 , 70), 55 (32), 43 (19), 41 (62);
e.e. 99%; [α]25=−4.4 (c 0.04, CH3OH); [Θ]234=+1543, [Θ]210=−1441 (CH3OH); UV (CH3OH), λmax
D
(ε, M−1 cm−1): 216 (89).
4.5.4. (−)-(3R,4aR,6R,8aS)-cis-6-(1,1-Dimethylethyl)-3-methyl-octahydro-2H-1-benzopyran-2-one 14
1
Mp 87–88°C (from light petroleum); IR (Nujol): 1740 (–O–C_O); H NMR, δ, ppm: 4.44 (1H, bq,
WH, 8.0, H-8a), 2.60 (1H, m, H-3), 2.42 (1H, dt, J1=J2 9.5, J3 13.7, H-4), 2.15 (1H, m, H-8), 1.97 (1H,
m, H-6), 1.54 (3H, m), 1.31 (1H, m), 1.18 (3H, d, J 6.8, CH3), 1.00 (2H, m), 0.84 [9H, s, C(CH3)3]; 13
C
NMR, δ, ppm: 176.8 (s, C-2), 74.6 (d, C-8a), 47.1 (d, C-6), 34.1 (t, C-4), 33.8 (d, C-4a), 32.3 (d, C-3),
32.0 [s, C(CH3)3], 30.9 (t, C-5), 30.1 (t, C-8), 27.3 [3q, (CH3)3], 20.7 (t, C-7), 15.5 (q, CH3); MS, m/z:
209 (M−CH+3 , 2), 168 (95), 150 (10), 123 (11), 95 (100), 87 (46), 81 (27), 67 (25), 57 (38), 55 (31); 99%
e.e.; [α]25=−34.3 (c 0.14, CH3OH); [Θ]216=−11909 (CH3OH); UV (CH3OH), λmax (ε, M−1 cm−1): 214
D
(172).

C. Forzato et al. / Tetrahedron: Asymmetry 10 (1999) 1243–1254
1253
4.5.5. (+)-(4aR,6R,8aS)-cis-3,3-Dimethyl-6-(1,1-dimethylethyl)-octahydro-2H-1-benzopyran-2-one 15
Mp 103°C (from light petroleum); elemental analysis: calculated: C, 75.58%; H, 10.99%; found: C,
1
74.4%; H, 11.0%; IR (Nujol): 1740 (–O–C_O); H NMR, δ, ppm: 4.60 (1H, bq, H-8a), 2.13 (1H, m,
H-8), 2.04 (1H, dd, J1 7.3, J2 14.2, H-4), 1.88 (1H, m, H-6), 1.66–1.34 (5H, m), 1.32 (3H, s, CH3), 1.30
(3H, s, CH3), 1.11 (2H, m), 0.85 [9H, s, C(CH3)3]; 13C NMR, δ, ppm: 178.9 (s, C_O), 76.6 (d, C-8a),
47.8 (d, C-4a), 41.8 (t, C-4), 36.4 (s, C-3), 35.1 (d, C-6), 32.7 [s, C(CH3)3], 31.3 (t, C-8), 30.9 (q), 30.3
(q), 28.5 (t, C-5), 27.7 [q, (CH3)3], 20.6 (t, C-7); MS, m/z: 223 (M−CH+3 , 2), 182 (M−C4H8+·, 15), 137
(20), 126 (37), 94 (65), 88 (23), 82 (21), 81 (37), 80 (21), 69 (22), 67 (21), 57 (100), 55 (29), 43 (20),
41 (56); 99% e.e.; [α]25=+4.9 (c 0.08, CH3OH); [Θ]221=−4204 (CH3OH); UV (CH3OH), λmax (ε, M−1
D
cm−1): 212 (194).
Acknowledgements
The authors thank the M.U.R.S.T. and C.N.R., Rome, and the University of Trieste for financial
support.
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