The compound of formula (XV) is reacted with a suitably substituted aldehyde of formula (V), wherein R3A is as previously defined, in the presence of a reducing agent such as sodium cyanoborohydride, sodium triacetoxyborohydride, and the like, under dehydrating conditions, for example, in an acid alcohol solution such as acidic methanol or in a solution of titanium tetraisopropoxide in DCM, to pro