Strategic use of pinacol-terminated Prins cyclizations in target-oriented total synthesis

Intramolecular Diels−Alder Cycloadditions of Fulvenes. Application to the Kigelinol, Neoamphilectane, and Kempane Skeletons

A variety of polycyclic ring skeletons (e.g., kigelinol, neoamphilectane, and kempene systems) can be prepared rapidly via intramolecular Diels-Alder cycloadditions (IMDA) of fulvenes. The length of the tethers and the diversity of the substituents on the fulvene core dictate the nature of the IMDA pathway. [reaction: see text]

An Efficient Sequential Reaction Process to Polysubstituted Indolizidines and Quinolizidines and Its Application to the Total Synthesis of Indolizidine 223A

The reaction of iodides 1 with delta-chloropropylamines 5 in MeCN assisted with K2CO3 undergoes a sequential S(N)2/Michael addition/SN2/SN2 reaction process to give polysubstituted indolizidines and quinolizidines. Using this method, indolizidine 223A is synthesized from 2-ethyl-2-hexenoic acid in 12 linear steps and 14.5% overall yield. [Reaction: see text]

Axial-selective prins cyclizations by solvolysis of alpha-bromo ethers

Prins cyclizations are intramolecular electrophilic additions of oxocarbenium ions. They lead to tetrahydropyrans with a heteroatom at the 4-position, and usually show moderate-to-high selectivity for equatorial substitution. We have found that Prins cyclizations carried out under specific conditions produce tetrahydropyrans with almost exclusive formation of the axial 4-substituent. TMSBr, AcBr, and TMSI all lead to axial-selective Prins cyclizations with alpha-acetoxy ether substrates in the presence lutidine. The mechanism appears to involve solvolysis of the intermediate alpha-bromo ether rather than specific or Lewis acid-catalyzed rearrangement. The scope of the reaction, the high yields, and the stereoselectivity make this a valuable new method for tetrahydropyran formation.

Mechanistic Studies of the Biomimetic Epoxy Ester−Orthoester and Orthoester−Cyclic Ether Rearrangements

The relative rates of acid-catalyzed rearrangements of epoxy esters to [3.2.1]bicyclic orthoesters, the subsequent rearrangements of these ortho esters to substituted tetrahydrofurans, and the rates of orthoester hydrolysis at pH 4.75 were measured in NMR kinetics experiments. The ease of formation and stabilities of these orthoesters compared favorably with the OBO-type [2.2.2]bicyclic orthoesters typically used as protecting groups of carboxylic acids. Studies with 13C NMR-detected 18O-labeling show that epoxy ester rearrangement takes place preferentially via 6-exo cyclization, although the 7-endo process competes when the distal center of the epoxide is disubstituted. The ortho ester-cyclic ether rearrangement was shown by 18O-labeling to occur exclusively via intermediacy of a five-membered dioxonium ion. The structures of the hydrolysis products also indicate the intermediacy of a dioxolanium ion during hydrolysis. The implications for a hypothetical biosynthesis of marine polyether toxins are discussed.