Mushrooms of the genus Retiboletus contain the retipolides A-D (1, 6-8), unusual 14-membered spiromacrolides with a biphenyl ether linkage. The structures of these metabolites suggested a biogenetic sequence starting from retipolide E (16), which could furnish the unique 2(3H)-oxepinone unit of retipolide C (7) via an oxidative enlargement of the 4-hydroxyphenyl ring. A subsequent O/C-acyl shift would then lead to the cyclopenta[c]pyran system of retipolide A (1). The proposed precursor 16 was synthesized and subsequently detected in the fungal extract in addition to the probable biosynthetic intermediates butyrolactone II (13), tyrosol (14), and secoretipolide E (15). The structural elucidation of retipolide A (1) was accomplished after transformation into a methoxyfulvene derivative 2a, and the absolute (R) configuration of 1 was assigned by a single-crystal X-ray analysis of the corresponding (S)-sec-butoxyfulvene analogue 2b. Some samples of R. retipes/ornatipes contained isoretipolide A (9), an isomer of 1 with a highly strained 12-membered lactone ring incorporating two C,C-coupled 4-hydroxyphenyl units. A comparison of the CD spectrum of retipolide A (1) with those of the other retipolides and isoretipolide A (9) indicates that all of these macrolides possess R configuration.