Diels‐AlderRegioselectivity Controlled by Remote Substituents. The Cycloadditions of 1‐(Dimethoxymethyl)‐2,3‐dimethylidene‐ and ‐2,3,5,6‐tetramethylidene‐7‐oxabicyclo[2.2.1]heptanes

Abstract

The syntheses of 2,3-dimethylidene-and 2,3,5,6-tetramethylidene-7-oxabicyclo[2.2. llheptanes substituted in position C(1) are reported. The 1-dimethoxymethyl group in derivatives 2 and 6 controls the regioselectivity of the Lewis-acid-catalyzed Diels-Alder additions with methyl vinyl kctone and butynone. For the EtAIC1,-catalyzed addition of methyl vinyl ketone to 6, the regioselectivity can be reversed by a small solvent modification. The tetraene 2 is a versatile reagent for regioselective 'tandem' cycloadditions.

Anodic Cyclizations, Densely Functionalized Synthetic Building Blocks, and the Importance of Recent Mechanistic Observations

“…17 However, in the current case, the reaction led to no conversion of the dimethoxy acetal to the aldehyde. Efforts using trifluoroacetic acid 18 or potassium ferrocyanide and LiBF 4 19 to cleave the dimethoxy acetal proved to be equally problematic. Each reaction attempted led to the complete recovery of the starting material.…”

Asymmetric Synthesis and DNA Intercalation of (−)-6-[[(Aminoalkyl)oxy]methyl]-4-demethoxy-6,7- dideoxydaunomycinones1,

“…However, when methyl vinyl ketone was coordinated to a strong Lewis acid some selectivity could be observed, the best results being obtained for EtAlCl 2 . It was found also that the regioselectivity (product ratio) depends on the solvent . A single monoadduct ( 17a ) was obtained for the cycloaddition of 15 to 1-acetylvinyl p -nitrobenzoate precomplexed with BF 3 ·Et 2 O .…”

“…There are 16 modes of addition corresponding to the “para” (→ 17 + 18 ) and “meta” (→ 19 + 20 ) orientations, to the “Alder” or “anti-Alder” rule and whether the exo or endo face of the bicyclic exocyclic diene is involved. Under thermal conditions the Diels−Alder addition of methyl vinyl ketone to 15 is not stereoselective and gives a mixture of all possible monoadducts . However, when methyl vinyl ketone was coordinated to a strong Lewis acid some selectivity could be observed, the best results being obtained for EtAlCl 2 .…”