[anilsuri]

Could anyone tell me if there any instances of Wittig reagents cycloadding across an unsaturated C=C bond?  Thanks.

[barcrphd]

refer jerry march latest addition. it is a wonderful book on organic chemistry and so many cases are discussed. you may get some referece from it which will be useful for you.

[wereworm73]

Interesting question, but I doubt it.  The ylide might do a nucleophilic attack on an electrophilic carbon from a C=C pair, but that carbon would need an oxygen for the oxaphosphetane intermediate to occur.  If that oxygen is part of an -OH group, you'd have an enol which tautomerizes to an aldehyde or ketone, which just sets up a carbonyl for the ylide to attack instead.  If it's in an -OR group, then the R would have to break off for the betaine ion to occur (which doesn't seem very likely).