Synthesis of Substituted Tetrahydropyrans via Intermolecular Reactions of δ-Halocarbanions with Aldehydes

Abstract

Intramolecular substitution in δ-halocarbanions leading to cyclobutanes is a relatively slow process, thus they readily add to carbonyl groups; the thus-produced anionic adducts cyclize to tetrahydropyran derivatives. A simple mechanistic discussion, optimization of the reaction conditions, and scope of the reaction is presented.

Key words

aldol reactions - sulfones - halocarbanions - cyclizations - tetrahydropyrans

References

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• 10 For example of reactions of nonstabilized δ-halocarbanion equivalents, see: Mudryk B. Cohen T. J. Am. Chem. Soc.  1991,  113:  1866 
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• The erythro-isomer gave product 3a exclusively, while the threo-isomer gave a mixture of 3a/3b (9:1, according to ¹H NMR). This observation may lead to the conclusion, that erythro-isomer cyclizes relatively rapidly, while this process is slower for the threo-isomer and competitive retro-aldol reaction gives cross product 3b. The conformational preference for the cyclization of diastereomers of analogous aldol-type adducts 2a on the basis of their ¹H-¹H coupling constants and reactivity pattern were discussed in:
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• 19 During the optimization process, we observed that excess benzaldehyde (>1.25 equiv) inhibits the second step of the reaction(cyclization), which causes contamination of product 3a with aldol-type adducts 2a and decreases the reaction yield. We assume that this effect is based on interaction of the O-anion of 2a with the carbonyl group of excess aldehyde and formation of a hemiacetal-type adduct. This type of equilibrium operates, for example, in the reaction of the anion of 2-chloroethanol with aldehydes: Barbasiewicz M. Mąkosza M. Org. Lett.  2006,  8:  3745 
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9

Fedoryński M., manuscript in preparation.

15

The behavior of 1a and 1b without an electrophile under basic conditions [t-BuOK (2 equiv), THF, -50 °C or 0 °C, 1 h] revealed that, in contrast to γ-halocarbanions, intramolecular substitution in δ-halocarbanions leading to cyclobutanes is a slow process disturbed by competitive elimination and oligomerization reactions.

16

The ratio of the diastereomers of 2a remains almost constant, in the range 1:0.55-1:0.70 (erythro/threo, according to ¹H NMR), during the course of the reaction.

18

In an independent experiment we performed the reaction of PhCHO (1 mmol), 4-MeOC₆H₄CHO (1 mmol), and 1b (1 mmol) under standard conditions to evaluate the effect of the relative electrophilicity of aldehydes. This experiment gave an approximately 1: 1 mixture of 3a and 3b (according to ¹H NMR of the crude reaction mixture), leading to the conclusion that complete equilibration of adduct 2a with 4-MeOC₆H₄CHO in the aldol dissociation-addition sequence should lead to an equimolar mixture of 3a and 3b.

20

Reaction of 1c with benzaldehyde (-40 °C, 1 h) led to a mixture of the expected product 3f and uncyclized aldol-type adduct 2f (according to ¹H NMR analysis of the crude reaction mixture). To force the cyclization process the temperature was increased to -25 °C. Similar behavior was observed for analogous reactions of γ-halocarbanions: an aldol-type adduct of 3-chloropropyl phenyl sulfone carbanion and benzaldehyde cyclizes much faster to the tetrahydrofuran derivative than its ester or cyano congeners: Barbasiewicz M., Mąkosza M., unpublished results.

21

Probably due to the less favorable equilibrium of addition of stabilized enolate of ketone to the carbonyl group under these conditions, as compared to other less stabilized carbanions, see ref. 2 for details. The only isolable compound was the product of reaction of the expected tetrahydropyran derivative with the second molecule of aldehyde and/or its subsequent transformations (yield ˜20%).