Addition Reactions of Activated Synthons

Pro-azaphosphatranes lb or Id catalyze the reaction of activated allylic compounds with aromatic aldehydes to produce either a Baylis-Hillman product or a j8,y-unsaturated 1,2-addition product, depending on the type of allylic compound employed [131]. As seen in Eqs. (18) and (19), the activated allylic compounds shown react efficiently with aromatic aldehydes in the presence of 20 to 40 mole percent of lb or Id to give exclusively a-addition products. Such products are useful intermediates for the synthesis of substituted tetrahydrofurans through base-promoted electrophilic cyclizations. To our knowledge there have been only a few reports in which allylic cyanides and esters of the type used here have been utilized in 1,2-addition to aldehydes [131]. In those reports, an ionic base was used, y addition was observed, or a mixture of a- and y-addition products were obtained. The latter two observations contrast ours in which exclusive a-addition occurred. 

Allyl cyanide on the other hand, Eq. (20), results in an allylic transposition affording only the Baylis-Hillman product. The Baylis-Hillman reaction has long been of interest. Generally, high pressure is required to induce such a reaction and an amine such as DABCO as well as lengthy reaction times (1-4 weeks) are usually required [131]. The transformation shown in Eq. (20) advantageously affords this product under very mild conditions and in very short reaction times compared with both older as well as more recent literature approaches. Furthermore, this reaction is successful with aromatic aldehydes that have generally led to unreliable results under typical Baylis-Hillman conditions. 

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