Synthesis of polycyclic structures by the Diels-Alder reaction of inner-outer-ring 1,3-bis(trimethylsilyloxy)dienes

A Diels-Alder reaction of novel inner-outer-ring 1,3-silyloxydienes 5-8 with a variety of dienophiles to afford highly functionalized polycyclic structures is reported. The inner-outer-ring 1,3-silyloxydienes 5-8 containing five- to seven-membered carbocyclic and heterocyclic rings were prepared in a single reaction vessel from 2-acetylcyclocarbonyls in quantitative yields. The Diels-Alder reaction with 1,4-benzoquinone (BQ), dimethyl acetylenedicarboxylate (DMAD), and methyl vinyl ketone (MVK) proceeded smoothly at room temperature, affording functionalized polycyclic naphthols, phenols, and enones with high regioselectivity and good yields (39-75%). Moreover, dienes 5-8 also reacted in a hetero-Diels-Alder reaction with benzaldehyde (BA) and N-benzylideneaniline (NBA) in the presence of catalytic amounts of ZnCl2, affording substituted polycyclic pyranones and pyridinones in good yields (40-93%). Overall, our synthetic strategy provides straightforward access to an interesting set of polycyclic structures useful for natural and nonnatural product synthesis.     

Palladium-catalyzed aryl-aryl cross-coupling reaction using ortho-substituted arylindium reagents

A range of biaryl compounds (aryl-aryl, aryl-heteroaryl, and heteroaryl-heteroaryl) can be efficiently prepared by a palladium-catalyzed cross-coupling reaction between ortho-substituted triarylindium reagents and aryl halides. The triarylindium reagents are prepared by directed ortho-lithiation and transmetallation to indium from the corresponding benzene derivatives using various directed metallation groups (DMGs). The reaction proceeds smoothly in high yields and short reaction times with high atom economy (the three aryl groups attached to indium are efficiently transferred).     

Stereoselective convergent synthesis of 24,25-dihydroxyvitamin D3 metabolites: a practical approach

Vitamin D3 active metabolites 24R,25-(OH)2-D3, 24S,25-(OH)2-D3, and 1 alpha, 24R,25-(OH)3-D3 were synthesized by a convergent and stereoselective approach. In the synthetic route, the stereogenic center at C-24 was generated through ultrasonically induced aqueous conjugate addition of iodide 6 to Seebach's dioxolanone 5, and the vitamin D triene system was constructed using the Lythgoe approach. The synthesis, which is both short (seven steps from iodide 6) and efficient (32-40% overall yield), allows the preparation of large quantities of the metabolites and provides a novel example of a highly stereoselective reaction promoted by the zinc-copper couple in aqueous media.     

Practical and efficient enantioselective synthesis of alpha-amino acids in aqueous media

Enantiomerically pure natural and unnatural alpha-amino acids have been synthesized from a chiral methyleneoxazolidinone by means of a highly diastereoselective 1,4-conjugate addition of alkyl iodides in aqueous media. The zinc-copper conjugate addition reaction exhibits high chemoselectivity, with the possibility of using functionalized iodides, to afford a single diastereomer in short reaction times and with good yields.     

Nonsymmetrical 3,4‐Dithienylmaleimides by Cross‐Coupling Reactions with Indium Organometallics: Synthesis and Photochemical Studies

The synthesis and photochemical study of novel nonsymmetrical 1,2‐dithienylethenes (DTEs) with a maleimide bridge have been carried out. The synthetic approach to the DTEs was based on successive selective palladium‐catalyzed cross‐coupling reactions of 5‐susbtituted‐2‐methyl‐3‐thiophenyl indium reagents with 3,4‐dichloromaleimides. The required organoindium reagents were prepared from 2‐methyl‐3,5‐dibromothiophene by a selective (C‐5) coupling reaction with triorganoindium compounds (R₃In) and subsequent metal–halogen exchange. The coupling reactions usually gave good yields and have a high atom economy with substoichiometric amounts of R₃In. The results of photochemical studies show that these novel dithienylmaleimides undergo a photocyclization reaction upon irradiation in the UV region and a photocycloreversion after excitation in the visible region, thus they can be used as photochemical switches. ON–OFF operations can be repeated in successive cycles without appreciable loss of effectiveness in the process.     

A prototype of a rationally designed chemically powered Brownian motor

The design and development of a molecular system functioning as a prototype of a Brownian motor is presented. The road towards the molecular motor is illustrated with previous attempts preparing other mechanical devices, such as a molecular brake and a molecular ratchet. Thermal and chemical energy are used to achieve unidirectional rotation in the Brownian motor. The similarities of synthetic molecular devices with biological – microscopic – and mechanical – macroscopic – systems and the perils of extrapolating macroscopic principles to the molecular level are also discussed. Received: 15 November 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002