Oxazaphosphorinanes, conformation

A high degree of asymmetric induction has been realized in the carbanion-accelerated Claisen rearrangement of phosphorus-stabilized anions. Treatment of 1,3,2-oxazaphosphorinane (166) with freshly prepared lithium dimsylate led to a 95 5 ratio of a-methyl ketones (167) and (168) (Scheme 33). Li coordination combined with steric interactions provide the necessary control elements for stabilization of the highly organized allyl anion conformation (169). 

A series of 2-alkyl-, 2-alkanoyl- and 2-aroyl-2-oxo-l,3,2-oxazaphos-phorinanes bearing N-benzyl, N-benzhydryl and N-trityl substituents (294) have been obtained and their conformational preferences in solid state analysed. All 2-alkanoyl- and 2-aroyl-2-oxo-l,3,2-oxazaphosphorinanes had a gauche or anti relationship between the P = O and C = O bonds. This is presumably to minimise dipolar interactions between the two bonds as observed in a-diketones. ... 

A series of 1,3,2-oxazaphosphorinanes (62) has been prepared for conformational studies. Some new seven-membered ring phosphoramidites (63) and diamidites (64) have been synthesised. 

In the solid state and in solution, six-membered tetrahedral phosphorus compounds generally adopt a chair as the most stable ground-state conformation. However, Bentrude has shown, by nmr analysis, that the barrier to chairs twist interconversion is low in some six-membered systems (Day et al, 1984 Bentrude et al, 1984 Beres et al., 1987 Nelson et al., 1987). The twist/chair activation barrier for 1,3,2-oxazaphosphorinanes is low. Compound [79] exists in the twist conformation in the solid state and solution. Compound [80] adopts a full boat conformation, from solution nmr spectroscopy and crystal-structure analysis. 

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