A facile access to 2,3,6-trisubstituted 2H-pyran-5-carboxylates is developed by employing 2-alkyl-2-enals as reactants with acetoacetates. The reaction involves Knoevenagel condensation followed by a 6π-electrocyclization, in which the presence of the C2 alkyl substituent in the enals favors the

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Cyclization of disubstituted enals with acetoacetates in the system piperidine—acetic acid provides access towards trisubstituted pyrancarboxylates, e.g. (III) and (IX).

Regioselective TEMPO Oxidation of 2-Alkylidene-1,3-propanediols to (E)-2-Hydroxymethyl-2-alkenals and Application to 4-Alkylidene-2-penten-5-olide Synthesis. -An improved method for the oxidation of title alkylidenepropanediols (I) to monoaldehydes is presented, involving a combination of diacetoxyiodobenzene as terminal oxidant and benzyloxy-TEMPO as catalyst. The reaction proceeds with good E-selectivity for trisubstituted alkylidenepropanediols, but the E/Z-selectivity is lost for tetrasubstituted enediol (If). The formed hydroxymethylalkenals can be converted to title alkylidenepentenolides [cf. (VIII)] via two independent pathways. -(PENG, W.; ASHIDA, K.; HIRABARU, T.; MA, L.-J.; INOKUCHI*, T.; Tetrahedron 66 (2010) 51, 9714-9720, http://dx.