The diastereoselective syntheses of enantiopure Benzo- and Naphtho-pyrans related to the Aphid Insect Pigments

An overview is presented of synthetic endeavours directed towards the assembly of the enantiopure quinone A and quinone A, natural derivatives of the aphid insect pigments protoaphin- fb and protoaphin- sl . These are achieved through intramolecular diastereoselective cyclization of tethered phenolic lactaldehydes. The conformational implications of the cyclization process are discussed. Model reactions allow the formulation of a more concise, convergent route to these natural derivatives through the corresponding intermolecular reactions, in which the required complementary diastereoselectivity is provided by the choice of either titanium or magnesium phenolates for reaction with the ethoxyethyl-protected asymmetric lactaldehyde