Chromane-2-carboxaldehyde

Asymmetric synthesis of a chroman. Solladie and Moine have effected an en-antiospecific synthesis of the chroman-2-carboxaldehyde 7, a key intermediate in the synthesis of a-tocopherol, from (R)-( +)-l. The phosphonate 2, derived from 1, undergoes a Wittig-Horner reaction with the dimethyl ketal of pyruvaldehyde to afford the optically active vinyl sulfoxide 3. Condensation of the aldehyde 4 with the lithio derivative of 3 affords, after silyl deprotection, the allylic alcohol 5 as the only diastereoisomer. This... 

SoUadie and Moine have reported the highly diastereoselective reaction between the a-lithiated-a,P-unsaturated sulfoxide (51) and the aromatic aldehyde (52), giving (53) as the predicted product diastereoisomer as part of the enantioselective synthesis of the (S)-chroman-2-carboxaldehyde (54) (Scheme 5.17) [55,56]. The high diastereoselectivity is particularly notable in view of the much smaller effects observed using similar systems [52]. 

The reaction of salicylaldehydes with a,P-unsaturated aldehydes in aqueous dioxane and under sonification affords a mixture of 2//-[l]benzopyran-3-carboxaldehydes and a tricyclic hemiacetal. The former is considered to arise from the 1,4-addition of a phenolate ion and the latter by a vinylogons aldol reaction. The choice of base controls the relative amounts of the two products, with Na2C03 favouring the chromene and NEts the chroman. <05ASC555>. 

L-Proline is an efficient catalyst for the reaction between 2 -hydroxyacetophenones and aryl and heteroaryl carboxaldehydes which yields a mixture of chalcone and chroman-4-one cyclic ketones afford only the 2-spiro-linked chromanone <05TL6991> and a lipase-catalysed reaction introduces asymmetry in a multistep synthesis of 3-benzylchromanones <05H(65)761>. 

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