Photochemistry of 6,6‐Dimethyl‐ and 2,2,6,6‐Tetramethyl‐2 H ,6 H ‐pyran‐3‐one

The title compounds 1a and 1b have been synthesized in two steps from the saturated pyran‐3‐ones 2a and 2b , respectively. Upon irradiation (254 nm or 350 nm) in dilute solutions (10 −3 −10 −2 M ), compounds 1 undergo a formal [4 + 2] cycloreversion from the excited triplet state to give (2‐methylprop‐1‐enyl)ketene ( 11 ) and either formaldehyde or acetone, ketene 11 being trapped by H 2 O or MeOH to afford 4‐methylpent‐3‐enoic acid ( 5 ) or its methyl ester 4 in 75–85% isolated yield. In this (monomolecular) photoreaction, heterocycles 1 differ from their alicyclic counterparts, i.e. , 4,4‐dimethylcyclohex‐2‐enone ( 10a ) and 4,4,6,6‐tetramethylcyclohex‐2‐enone ( 10b ), as no rearrangement to a 4‐oxabicyclo[3.1.0]hexan‐2‐one occurs. On the other hand, the photochemical behavior of pyranone 1a in bimolecular reactions (cyclodimerization, [2 + 2] cycloaddition to 2,3‐dimethylbut‐2‐ene) resembles that of enone 10a .