Alkenyl-substituted cyclic acetals

The reaction of a halide with 2-butene-1,4-diol (104) affords the aldehyde 105, which is converted into the 4-substituted 2-hydroxytetrahydrofuran 106, and oxidized to the 3-aryl-7-butyrolactone 107[94], Asymmetric arylation of the cyclic acetal 108 with phenyl triflate[95] using Pd-BINAP afforded 109, which was converted into the 3-phenyllactone 110 in 72% ee[96]. Addition of a molecular sieve (MS3A) shows a favorable effect on this arylation. The reaction of the 3-siloxycyclopentene 111 with an alkenyl iodide affords the. silyl... 

The reaction of alkenyl alkyl ethers and ketene acetals with PX3 (X = Cl or Br) has been reported to occur readily in the presence of an organic base and to result in the electrophilic substitution of a vinyl hydrogen atom with the PX2 group. Thus, (2-alkoxyalkenyl)-, (l-bromo-2-alkoxyalkenyl)- and (2,2-dialkoxyalkenyl)-phosphorus dichlorides and dibromides were obtained in 70-98% yield. The reaction proceeds regio- and stereo-selectively and is believed to involve formation of a cyclic phosphirenium ion.74... 

Electrophilic substitutions of alkenyl-, aryl-, and alkynylsilanes with heteroatom-stabilized cationic carbon species generated by the action of a Lewis or Brpnsted acid (acyl cation, oxocarbenium ion, etc.) provide powerful methods for carbon-carbon bond formation. Particularly, intramolecular reactions of alkenylsilanes with oxocarbenium and iminium ions are very valuable for stereoselective construction of cyclic ether and amine units.21-23 For example, the BFj OEt -promoted reaction of (E)- and (Z)-alkenylsilanes bearing an acetal moiety in the alkenyl ligand gives 2,6-disubstituted dihydropyrans in a stereospecific manner (Scheme l).23 Arylsilanes also can be utilized for a similar cyclization.24... 

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