Lewis Acid-Promoted, Stereocontrolled, Gram Scale, Diels-Alder Cycloadditions of Electronically Matched 2-Pyrones and Vinyl Ethers: The Critical Importance of Molecular Sieves and the Temperature of Titanium Coordination with the Pyrone

Insight into the mechanism of the asymmetric addition of alkyl groups to aldehydes catalyzed by titanium-BINOLate species

The asymmetric addition of alkyl groups to aldehydes catalyzed by BINOLate-titanium complexes has become the testing grounds to evaluate the potential of new BINOL-based ligands. We have investigated the mechanism of this reaction and report our findings here. Model systems for the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), based on mono-oxygen-alkylated BINOL ligands have been examined. Comparison of the reactivity and enantioselectivity of the mono-alkyl BINOL derivatives with those of BINOL indicate that the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), is not active in the asymmetric addition reaction. Several BINOLate-titanium complexes have been synthesized and characterized by X-ray crystallography. These include the dinuclear (BINOLate)Ti(O-i-Pr)(2).Ti(O-i-Pr)(4), which contains a bridging naphtholate and isopropoxy group, trinuclear (BINOLate)Ti(O-i-Pr)(2).[Ti(O-i-Pr)(4)](2), and trimeric [(BINOL)Ti(O-i-Pr)(2)](3). The solid-state and solution structures reported here indicate that (BINOLate)Ti(O-i-Pr)(2) prefers to bind to titanium tetraisopropoxide rather than to itself, explaining why no nonlinear effects are observed in the catalytic reaction. Additionally, experimental evidence suggests that the BINOLate-titanium species responsible for the catalytic and stoichiometric asymmetric addition reactions are different, indicating that the proposed intermediate, (BINOLate)Ti(R)(aldehyde)(O-i-Pr), is not involved in either of these processes. Reactions were examined using different sources of the alkyl group [ZnMe(2) or MeTi(O-i-Pr)(3)]. Under similar conditions, it was found that the product ee's were the same, independent of whether ZnMe(2) or Me-Ti(O-i-Pr)(3) was used as the source of the alkyl groups. This indicates that the role of the dialkylzinc is not to add the alkyl group to the carbonyl but rather to transfer the alkyl group to titanium. On the basis of these results, we hypothesize that the intermediate in the asymmetric addition involves (BINOLate)Ti(O-i-Pr)(2)(aldehyde).MeTi(O-i-Pr)(3).

2,2-Disubstituted analogues of the natural hormone 1 alpha,25-dihydroxyvitamin D(3): chemistry and biology

Six new 2,2-disubstituted analogues of the natural hormone calcitriol have been prepared. Chemical novelty includes (1) the first example of an inverse-electron-demand Diels-Alder cycloaddition using a pyrone diene and a difluorinated vinyl ether dienophile, leading to difluorinated analogues 7 and (2) a conceptually streamlined approach to dimethylated 19-nor analogues. Analogues 7a and are similar to calcitriol in terms of in vitro antiproliferative activity, but they are different from calcitriol in terms of transcriptional activity: difluorinated analogue 7a is 2-3 times more active transcriptionally than calcitriol, whereas dimethylated analogue is 7.5 times less active transcriptionally. Whereas the in vivo calcemic activity of difluorinated analogue 7a is similar to that of calcitriol, dimethylated analogue is considerably less calcemic than calcitriol. Dimethylated analogue strongly suppresses parathyroid hormone (PTH) secretion.

Antiproliferative Hybrid Analogs of the Hormone 1alpha,25-Dihydroxyvitamin D(3): Design, Synthesis, and Preliminary Biological Evaluation

The combination of 10-12 kbar pressure plus Lewis acidic zinc dichloride promotes highly regioselective and stereoselective Diels-Alder cycloaddition between 3-bromo-2-pyrone, prepared in a new way, and unactivated terminal alkene 4-(tert-butyldimethylsiloxy)-1-butene. The conjugate base of A-ring allylic phosphine oxide 20 adds chemospecifically to the C-8 keto group of some C-8,C-24-diketones to form directly metabolically resistant 24-oxo analogs of 1alpha,25-dihydroxyvitamin D(3) (calcitriol). Several of these new hybrid analogs are as efficacious in vitro as calcitriol at inhibiting growth of murine keratinocytes even at physiologically relevant 10-100 nanomolar concentrations.

A Novel Oxidizing Reagent Based on Potassium Ferrate(VI)(1)

A new, efficient preparation has been devised for potassium ferrate(VI) (K(2)FeO(4)). The ability of this high-valent iron salt for oxidizing organic substrates in nonaqueous media was studied. Using benzyl alcohol as a model, the catalytic activity of a wide range of microporous adsorbents was ascertained. Among numerous solid supports of the aluminosilicate type, the K10 montmorillonite clay was found to be best at achieving quantitative formation of benzaldehyde, without any overoxidation to benzoic acid. The roles of the various parameters (reaction time and temperature, nature of the solvent, method of preparation of the solid reagent) were investigated. The evidence points to a polar reaction mechanism. The ensuing procedure was applied successfully, at room temperature, to oxidation of a series of alcohols to aldehydes and ketones, to oxidative coupling of thiols to disulfides, and to oxidation of nitrogen derivatives. At 75 degrees C, the reagent has the capability of oxidizing both activated and nonactivated hydrocarbons. Toluene is turned into benzyl alcohol (and benzaldehyde). Cycloalkanes are also oxidized, in significant (30-40%) yields, to the respective cycloalkanols (and cycloalkanones). Thus, potassium ferrate, used in conjunction with an appropriate heterogeneous catalyst, is a strong and environmentally friendly oxidant.