Diastereoselective synthesis of pentasubstituted γ-butyrolactones from silyl glyoxylates and ketones through a double reformatsky reaction

Full text of DOI:10.1002/anie.200900215

Angewandte
Chemie
DOI: 10.1002/anie.200900215
Multicomponent Reactions
Diastereoselective Synthesis of Pentasubstituted g-Butyrolactones
from Silyl Glyoxylates and Ketones through a Double Reformatsky
Reaction**
Stephen N. Greszler and Jeffrey S. Johnson*
The prevalence of g-butyrolactone substructures in natural
products continues to stimulate interest in the development of
concise and selective methods for their preparation. The
assembly of g-butyrolactones that contain multiple stereo-
centers typically requires the synthesis of complex precursors
through specialized routes.[1] Modular assembly strategies
that circumvent this limitation would be welcome additions to
the synthetic toolbox. Herein, we report diastereoselective
reactions of Reformatsky reagents, silyl glyoxylates, and
ketones that provide densely functionalized pentasubstituted
g-butyrolactones containing three contiguous stereocenters.
The reactions collectively constitute a rare example of the
diastereoselective generation of vicinal stereogenic tertiary
alcohols through aldolization.[2]
their acyclic precursors were formed in low yield. We
speculated that moderation of the enolate reactivity might
be necessary and screened Reformatsky reagents under
standard conditions.[6–8] The use of zinc enolates in combina-
tion with the appropriate reaction temperature facilitated the
development of
a
workable experimental protocol
(Scheme 2). The hydroxysilane 8 was isolated as the predom-
Silyl glyoxylates are conjunctive reagents that participate
in coupling reactions initiated by hydrides and nonstabilized
carbon nucleophiles.[2c,3] We examined the use of enolates and
their equivalents in an effort to expand the range of
nucleophilic promoters in transformations based on silyl
glyoxylates. The projected transformation, outlined in
Scheme 1, involves aldol addition to the silyl glyoxylate 2 to
Scheme 1. Three-component lactone synthesis. Bn=benzyl, TBS=tert-
butyldimethylsilyl.
Scheme 2. Optimization of the double Reformatsky reaction. General
conditions for all reactions: enolate (1.5 equiv), ketone (2.0 equiv), 2
(1.0 equiv), [2]0 =0.05m in Et2O. See the Supporting Information for
further details.
expose, after a [1,2]-Brook rearrangement,[4,5] a new enolate 3
capable of a second aldol reaction with an aldehyde or ketone
electrophile. Lactonization would then provide g-butyrolac-
tone 5.
Initial experiments with magnesium and lithium enolates
provided complex product mixtures; the desired lactones or
inant product when the reaction was conducted at À208C.
This result reflects an inability of the initial zinc aldolate to
undergo Brook rearrangement at this temperature [Scheme 2,
Eq. (1)].[9] We took advantage of this finding by forming the
initial adduct between the Reformatsky reagent and the silyl
glyoxylate at À208C prior to the introduction of the ketone
electrophile. Once the initial reaction between the silyl
glyoxylate and the zinc enolate was complete, the addition
of the ketone, followed by warming to À108C for 30 min, gave
the desired product 7 in 33% yield; however, 8 was still the
major product [Scheme 2, Eq. (2)]. Optimal conditions
involved this stepwise addition of the reagents, with an initial
reaction temperature of À308C; the reaction mixture was
then warmed to room temperature and stirred at this temper-
ature for 1 h to afford the desired product in 73% yield along
[*] S. N. Greszler, J. S. Johnson
Department of Chemistry
The University of North Carolina at Chapel Hill
Chapel Hill, NC 27599 (USA)
Fax: (+1)919-962-2388
E-mail: jsj@unc.edu
[**] Funding for this research was provided by the National Institutes of
Health (National Institute of General Medical Sciences,
GM068443).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900215.
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Table 1: Scope of the reaction in terms of the Reformatsky reagent and
with only trace quantities of 8 [Scheme 2, Eq. (3)]. The impact
terminal electrophile.[a,b]
of the countercation on the facility of the Brook rearrange-
ment remains a point of interest and development.[9] The
present example appears to involve an equilibrating mixture
À
À
of C Si and O Si isomers: The warming of a solution of the
unrearranged zinc aldolate to room temperature in the
absence of a ketone electrophile resulted in a complex
mixture that contained both the hydroxysilane 8 and the
product derived from protonation of the enolate 3 in similar
amounts.
Product
Yield
[%]
d.r.
Product
Yield
[%]
d.r.
52
67
46
11:1
40
57
53
12:1
Subsequent experiments were directed at improving the
diastereoselectivity of the reaction. The use of the acetate-
derived Reformatsky reagent 1a in conjunction with aceto-
phenone led to modest diastereoselectivity [d.r. 3:1;
Scheme 2, Eq. (4)]; however, when the propionate reagent
1b was used to initiate the reaction, the desired lactone
product was obtained in 67% yield with d.r. > 25:1 [9b/all
other diastereomers; Scheme 2, Eq. (5)]. Similar results were
obtained with the Reformatsky reagent derived from ethyl
2-bromobutyrate [Scheme 2, Eq. (6)], but the use of the
analogous isovalerate led to a low yield and complex
diastereomer mixtures.[10] The results with 1b and 1c were
somewhat unexpected, as diastereoselectivities in Reformat-
sky reactions with simple ketones are generally modest.[11]
The use of other alkyl esters or other silyl groups in the silyl
glyoxylate reagent generally led to lower yields and/or
diastereoselectivities.[12]
An examination of a variety of alkyl aryl ketones revealed
favorable results within this subset of electrophile (Table 1).
The yields of the isolated products ranged from 40 to 73%
with diastereomer ratios from 7.5:1 to > 25:1. Surprisingly
high selectivity was also observed in the formation of 10 f with
benzoylthiophene. In contrast, the equivalent reaction with
benzaldehyde provided the product 10m with only 3:1
diastereoselectivity.
>25:1
>25:1
30:1
>25:1
73
70
41
7.5:1
18:1
9.5:1
40
>25:1
68
63
3:1
1.6:1
71
–
In cases in which modest yields were observed, the
quenched glycolate enolate 3 comprised the majority of the
mass balance; intermolecular proton transfer is a likely
pathway with certain ketone electrophiles.[10] The highly
substituted g-butyrolactones were purified conveniently in
most cases through selective crystallization from pentane
after column chromatography to afford the products as single
diastereomers.[12]
The high diastereoselectivity observed in the formation of
isobenzofuranone 10q (terminating electrophile: methyl
2-acetylbenzoate), which must result from lactonization via
a different transition state, suggests that selective lactoniza-
tion by one of a mixture of equilibrating stereoisomeric
aldolates is probably not responsible for the remarkable
diastereoselectivity. The alkyl group R1 (R1 = Me in
Scheme 3) in the substituted Reformatsky reagents is thus a
likely determinant of the facial selectivity in the second
Reformatsky reaction. In the present case, the ethyl ester
could conceivably enforce the illustrated boat/twist-boat
transition state[11a] through chelation. This type of organized
structure, 11, provides a plausible rationalization for the high
enolate facial selectivity insofar as an approach of the ketone
syn to the hydrogen atom a to the ester group should be
preferred.[13] The model shown in Scheme 3 further supposes
48
44
>25:1
>25:1
68
51
1.2:1
20:1
[a] Reagents: enolate (1.5 equiv), ketone (3.0 equiv), 2 (1.0 equiv), [2]0 =
0.05m in Et2O. [b] See the Supporting Information for detailed
procedures. Stereostructures were determined through NOESY experi-
ments.
a pseudoequatorial orientation of the aryl group and an E
enolate geometry (as in 12°); however, it would be premature
to discount alternative models (including those involving
other structures of the organometallic intermediate) in the
absence of more-complete experimental data.
The transformations shown in Scheme 4 further highlight
the synthetic utility of this methodology. Alkylation occurred
faster than dehydrohalogenation when 10h was heated with
DBU and resulted in the formation of the bicyclic lactone 13,
which contains three contiguous fully substituted stereogenic
centers. A zinc-insertion/elimination reaction of bromolac-
tone 10g provided the g,d-unsaturated acid 14, which can be
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transparent colorless crystals. Additional details and full character-
ization data are presented in the Supporting Information.
Received: January 13, 2009
Published online: April 16, 2009
Keywords: diastereoselectivity · ketones · lactones ·
.
Reformatsky reaction · silyl glyoxylates
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diazabicyclo[5.4.0]undec-7-ene.
viewed formally as a product of glycolate enolate alkenyla-
tion.
In summary, we have developed a highly diastereoselec-
tive route to pentasubstituted g-butyrolactones in the form of
a double Reformatsky reaction of propionate enolates with
silyl glyoxylates and aryl ketones. The moderate yields
observed in certain cases are offset by the high level of
structural complexity engendered: The reaction generates
three contiguous stereocenters and two carbon–carbon bonds
with unusually high diastereoselectivity. Second-stage trans-
formations of the product lactones further enhance their
synthetic utility.
Experimental Section
9b: A freshly prepared solution of the Reformatsky reagent 1b
(1.5 mL, 0.6 mmol, 1.5 equiv) was diluted with diethyl ether (4 mL),
and the resulting solution was cooled to À308C in an acetone/dry-ice
bath. (The bath temperature was monitored with a thermocouple
probe). The silyl glyoxylate 2 (112 mg, 0.4 mmol, 1.0 equiv) was
placed in an oven-dried vial. The vial was then purged with N2, and
diethyl ether (1 mL) was added. The resulting solution of 2 was added
dropwise to the solution of the Reformatsky reagent over 2 min with a
syringe. Additional diethyl ether (0.5 mL) was used to rinse the vial.
When the consumption of the silyl glyoxylate was complete (generally
10–15 min at À308C, as determined by TLC), acetophenone (6b;
0.140 mL, 1.2 mmol, 3.0 equiv) was added, and the reaction mixture
was allowed to warm to 08C in the acetone bath over 45 min. The
reaction mixture was then stirred at room temperature for 30 min,
diluted with diethyl ether (5 mL), and quenched with saturated
ammonium chloride (1 mL). The resulting mixture was stirred until a
clear solution was obtained. The organic layer was removed, and the
aqueous layer was extracted with diethyl ether (3 ꢀ 5 mL). The
combined organic extracts were washed with brine (5 mL), dried with
magnesium sulfate, and concentrated in vacuo. The residue was
purified by flash chromatography (hexanes/ethyl acetate 9:1), and the
product was crystallized from pentane to give 9b (121 mg, 69%) as
[10] Intermolecular proton transfer was detrimental in other reac-
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[12] See the Supporting Information for details.
[13] J. M. Humphrey, R. J. Bridges, J. A. Hart, A. R. Chamberlin, J.
Org. Chem. 1994, 59, 2467 – 2472.
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