Isopulegol undergoes Prins cyclization reaction in the presence of 20 wt% of acid-treated montmorillonite K10 to produce octahydro-2H-chromen-4-ols in good yields and with high cis selectivities under solvent-free conditions. The solid-acid catalyst can be reused without loss of its activity.
An efficient method has been developed for the synthesis of two new classes of tetrahydropyran derivatives comprising amide, tetrazole or benzothiazole moieties via a three-component reaction of 6-methylhept-5-en-2-ol, arylaldehydes and nitriles/thiols in the presence of a tetrafluoroboric acid diethyl ether complex. The reaction proceeds via the formation of an oxocarbenium ion. This protocol is highly diastereoselective and only single diastereomer has been isolated in each case.
The first example of the direct conversion of benzaldehydes into their corresponding N‐tert‐butyl amides through a Schmidt reaction/Ritter reaction sequence is described. A reagent mixture consisting of NaN3 and HBF4·OEt2 in acetic acid efficiently reacts with aromatic and conjugated (α,β‐unsaturated) aldehydes to produce nitrile derivatives, which in situ undergo a Ritter reaction with tert‐butyl acetate to afford the corresponding N‐tert‐butyl amides in almost quantitative yields. The method needs no column chromatography purification. The wide substrate scope as well as the ready availability of the reagent system also make this protocol convenient.
o-Quinonemethides generated by condensation of 6-methylhept-5-en-2-ol with various salicylaldehydes in the presence of 30 mol% of fluoroboric acid etherate or triflic acid undergo [4+2]-cycloaddition reaction in situ to afford angularly fused pyranochromene derivatives in good yields and excellent diastereoselectivities.
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