tereocontrolled syntheses of the C16-C28 CD-spiroacetal subunit I of altohyrtin A/spongistatin 1, relying on kinetic and thermodn. control of the spiroacetal formation, are described. The kinetic control approach resulted in a slight preference (60 : 40) for the desired spiroacetal isomer. The thermodn. approach allowed ready access to the desired spiroacetal I by acid-promoted equilibration, chromatog. sepn. of the C23 epimers and resubjection of the undesired isomer to the equilibration conditions. This scalable synthetic sequence provided multi-gram quantities of I, thus enabling the successful completion of the total synthesis of altohyrtin A/spongistatin 1, as reported in Part 4 of this series.