Ferruccio Bertolini

Recent Progress in the Synthesis of 2,3-Dihydrobenzofurans

Introduction ..........................................................................................386 I. Natural Occurrence of 2,3-Dihydrobenzofurans................................387 II. Synthesis by Dehydrative Methods .....................................................389 III. Radical Cyclizations and Electrocyclizations .....................................391 IV. Biomimetic Couplings and Cycloadditions .........................................393 V. Biomimetic Asymmetric Synthesis ......................................................397 VI. Synthesis via Benzyne ...........................................................................399 VII. Intramolecular Substitutions of ortho-Allylphenols without Metal Catalysts .........................................................................................401 VIII. Transition Metal-Catalyzed Processes ................................................402 1. Transition Metal-catalyzed Nucleophilic Substitutions............................402 2. Transition Metal-catalyzed Intramolecular Hydroalkoxylations ..............405 3. Transition Metal-catalyzed Insertion-Cyclizations ..................................407 4. Palladium-catalyzed Intramolecular Heck-type Cyclizations ...................409 IX. Anionic Cyclizations .............................................................................412 X. Miscellaneous Methods ........................................................................413 References..............................................................................................415

Regio- and Stereoselective Ring Opening of Enantiomerically Enriched 2-Aryl Oxetanes and 2-Aryl Azetidines with Aryl Borates

The regioselective ring opening of 2-aryl-substituted four-membered heterocyclic rings with phenols, including catechol, was achieved by the use of aryl borates in mild and neutral reaction conditions without the aid of any transition metal catalysts. While N-alkyl azetidines were found not to be reactive, optically active N-tosyl azetidines gave the corresponding beta-aryloxy amines in a racemic form, thus indicating the considerable carbocationic character of the transition state. The introduction of a hydroxyl group in the azetidine ring (i.e., an azetidinol), able to anchor the aryl borate and to direct the subsequent nucleophilic delivery, was shown to determine the ring-opening process with predominant inversion of configuration. When enantiomerically enriched 2-aryl oxetanes were used, the reduced extent of racemization observed (up to 93:7 er) was rationalized by an intramolecular delivery through a six-membered transition state, giving beta-aryloxy alcohols with a predominant retention of configuration (i.e., a syn-stereoselective ring opening). The aryloxy alcohols obtained, endowed with suitable functionalities, can be cyclized to give access to enantiomerically enriched 2-aryl-1,5-benzodioxepins.

Nickel-catalyzed borylative ring opening of vinyl epoxides and aziridines.

A mild ring opening of vinyl epoxides and aziridines with B(2)Pin(2) catalyzed by Ni(0)-Binap affords new functionalized allylic boron derivatives which undergo sequential transformations. The uncatalyzed allylation of aldehydes allows obtaining challenging bishomoallylic alicyclic 1,3-diols and 1,3-amino alcohols with remarkably high stereoselectivities. Valuable trans-bisallylic 1,4-amino alcohols can be obtained by a simple oxidation.

Regioselective copper-catalyzed alkylation of [2.2.2]-acylnitroso cycloadducts: remarkable effect of the halide of Grignard reagents.

Ring opening with organometallic reagents of [2.2.2]-acylnitroso cycloadducts, including an enantioselective kinetic resolution of these compounds, has been accomplished for the first time. By the careful choice of reaction conditions, it was possible to obtain new cyclohexenyl hydroxamic acids with complete anti-stereoselectivity and a nice regioalternating control. A remarkable effect of the halogen of the Grignard reagent was observed during ring opening.

Stereoselective Synthesis of 4-Amino-2,3-unsaturated-N-Cbz-imino-O-glycosides via New Diastereoisomeric N-Cbz-Imino Glycal-Derived Allyl N-Nosyl Aziridines

The glycosylation of alcohols by the new diastereoisomeric d,l-6-deoxy-N-Cbz-imino glycal-derived allyl N-nosyl aziridines 5 and 6 affords, after deprotection of the 4-(N-nosylamino) group, the corresponding 2,3-unsaturated-N-Cbz-imino-O-glycosides bearing a free amino group on C(4) through a completely 1,4-regio- and substrate-dependent stereoselective glycosylation process.