An unprecedented product shape/size selectivity in the hydrolysis/decarboxylation reaction sequences of chromene with formic acid within MCM-41 as a nanoreactor in the liquid phase is observed. Chromene is expected to form pyrimidine in the reaction with HCOOH. However, here, serendipitously, the product switches from pyrimidine to enol lactones and δ-keto acids inside the nanoreactor. The shape/size selectivity in the liquid phase is a highly desired phenomenon in organic synthesis. We prepared four different varieties of porous silica materials to obtain this product selectivity. Very high yields of these biologically relevant compounds surpass that achieved by existing methodologies. Again, though there are few reports of coumarin-fused enol lactones, this approach enables the easy synthesis of naphthalene-fused enol lactones. Moreover, this is the first generalized synthesis of δ-keto acids following a number of hydrolysis/decarboxylation/ring opening/tautomerization/decarboxylation steps. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim