1- Benzopyrylium from phenols

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

The clarification of the recyclization mechanism of 2-benzopyrylium salts into naphthols under the action of alkali permits some generalizations (low temperature at the beginning of reaction followed by heating, and the use of cosolvents that are unable to form stable adducts). This leads to an increase in the yield of this reaction, considered earlier (70MI1) as lacking preparative interest, similar to the formation of phenols from monocyclic pyrylium salts. Consider the example shown in Scheme 116. 

Organometallic carbon nucleophiles add to flavyUum salts,as do activated aromatics like phenol, and enolates snch as those from cyanoacetate, nitromethane " and dimedone, all very efficiently, at C-4. Cyanide and azide add to 2-benzopyryliums at C-1. ... 

Chromans. - 2,4-Diethoxychromans (78) (mainly cis) represent a new type of chroman and have been prepared in high yield from a phenol (but not a deactivated phenol) and malonaldehyde bis(diethyl acetal) in the presence of SnCU they are converted into the benzopyrylium salts (79), in excellent yield, by treatment with perchloric acid. A Wittig-Horner reaction of the chiral phosphoryl ( S)-sulphoxide (81) and the ketone (80) gave the chiral sulphoxide (82), which was cyclized by aqueous alkali to a mixture which contained 22% diastereoisomeric excess of (25 )-chroman (i )-sulphoxide. This was converted, in three steps, into the aldehyde (83), which is a useful synthon for a-tocopherol. ... 

---