Short, convergent, stereoselective syntheses of enantiopure 2-benzopyran-5,8-quinones related to the aphid insect pigments, the protoaphins

The enantiopure (1S,3S,4R)-, (1R,3S,4S)- and (1S,3S,4S)-3,4-dihydro-1,3-dimethyl-4-hydroxy-2-benzopyrans 16, 25 and 26 are prepared in two related reaction sequences, each in eight steps and good overall yield. The starting materials are 4-methoxyphenol 6 and (2S,1′R and 2S,1′S)-1′ ethoxyethoxypropanal 9, the latter providing the source of asymmetry from the chiral pool. The three key reactions involved in each sequence are either highly or completely diastereoselective