Abatractr
The synthesis of enantio-and diastereomerically pure (55)4bromo-5(Imenthyloxy)furan2(5H)-one (2) and a general method to obtain (5s)4alkylarnino-and 4alkylthio-5-(l-menthyloxy)furan-2(5H)-ons in very good yields are described.
5-Alkoxyfuran-2(5H)-ones are useful intermediates in organic synthesis, especially when they bear addttional functional groups. The preparation of 5-methoxyfuran2(5H)-one is readily achieved by photooxygenation of furan, fiuoic acid or furfural using methanol as solvent.' The application of this procedure to the synthesis of 3-or Csubstituted 5-methoxyfuran2(5H)-ones is limited because of the difftcult accessibility of the starting 3-substituted furans and the lack of regioselectivity of the prccess.* We have previously reported that halogen addition to 5-alkoxyhtran-2(5H)-ones followed by HX elimination under different experimental conditions, affords a regioselective synthesis of 3-and 4-halogenated alkoxyfuranones.3
We have also described tbat nucleophilic halogen substitution on 4-bmmofuranone 1 is a suitable method for the introduction of other functional groups, as alkylamino and alkylthio at C-4 of the furanone nucleus.'
Optically active y-substituted butenolides could be remarkably useful as chiral synthons for the synthesis of natural products.' Due to the high versatility of butenolides, there have been many reports on the preparation of optically active a$-unsaturated y-lactones6 They have been obtained both from natural sources and from synthetic compounds. Among the previously reported methods, most of them take multistep synthesis and it seems diffticult to obtain large amounts of the material for practical use.
We considered the asymmetric transacetalization of 5-methoxy4substituted furan-2(5H)-ones with an enantiomerically pure auxiliary alcohol, such as menthol, as an attractive way to obtain large quantities of the enantio-and diastereomerically pure corresponding 5-mentbyloxy-2(5H)-ones. However, all attempts to obtain the 5-(f-menthyloxy)-4-(pyrrolidin-l-yl)furan-2(5H)-one in moderate yield, by reaction of menthol with the 5-methoxy4(pyrrolidin-1-yl)furan-2(5H)-one, in the presence of boron trifluoride-dietbyl ether or toluene-p-sulfonic acid failed, in contrast with the previous results reported by Pelter". Feringa,' and us* on