Vinyl esters such as 2, in the presence of DABCO, undergo initial reaction with aldehydes, followed by nucleophilic attack on a second aldehyde molecule. Subsequent intramolecular transesterification affords the dioxanones 5 in high yields and high diastereomeric excesses.In our endeavours to find new areas of application for the reaction of "potential vinyl carbanions" with aldehydes'-' ), we have developed a novel cyclization reaction. The vinyl ester Z4), activated by the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO), reacts initially with acetaldehyde in the usual aldol manner. However, in this instancc, a further reaction occurs with a second molecule of the aldehyde, followed by an intramolecular transesterification to afford the substituted 1,3-dioxanone 5a (Scheme 1). This product is obtained in 87% chemical yield and has a diastereomeric excess of 84% ([a]E = -5.6 (CHC13, 2.1)).This novel reaction is versatile in that various alkyl substituents may be introduced to the 2-and 6-positions of the dioxanone. In addition, it is very much faster than the classical reaction reported by Baylis and Hillman5,6) (30 min as opposed to 7 d), and the alcohol released on cyclization is recovered as the optically pure compound. However, while high diastereomeric excesses are obtained, only low cnantiomeric excesses have been obtained to date.A critical factor in this cyclization reaction is the choice of acrylate ester. With the usual alkyl esters, e.g. methyl and ethyl, no cyclization occurs, and the reactions can take up to 7 d for completion". However, with the pantolactone ester 2, the reaction is complete within 30 min. With acrylate esters such as the lactate and mandelate, which also incorporate the a-hydroxy carbonyl system, the reaction times are also fast (24 h), but slower than for the pantolactone ester. This reaction thus provides easy entry into a series of highly functionalised 1,3-dioxan-4-ones by a route not previously explored.In order to determine the stereochemistry of 5 a it was examined at 500 MHz for nuclear Overhauser effects. These studies showed unambiguously that 5a adopts the conformation shown in
