Propanal Wittig reaction

The fluorophenol could be converted into 56 in four good steps but the insertion of the vinyl group to give 57 by formylation and a Wittig reaction went in only 18% and the cyclo-propanation with a diazoester and Cu(I) (chapter 30) gave poor selectivity in favour of the cis isomer of 57. Worse still, it was necessary to protect the phenol as a methyl ether and the removal of the methyl group, the last step, went in only 52% yield, wasting nearly half of all the material. 

One limitation of the Wittig reaction is that a mixture of alkene stereoisomers sometimes forms. For example, reaction of propanal (CH3CH2CHO) with a Wittig reagent forms the mixture of E and Z isomers shown. 

The last step is an aldol reaction between two aldehydes. The easiest way to do this is by a Wittig reaction but a specific enol of propanal would also be fine. 

The Wittig reaction between 6 and an excess of the acetoxy phosphonium salt 9 in the presence of KOH instead of NaOMe in propan-2-ol at -30°C leads to the (all- )-alloxanthin (117) and its (9Z)-isomer (Scheme 2). 

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