The interaction of dioxygen with various tetrahedral aluminum alkyls, (tBu)3Al.OEt2 (1), tBu2Al(mu-OtBu)2AltBu2 (6), (tBu)2Al(mesal) (2) [mesal=methyl salicylate anion], R2Al(mu-pz)2AlR2 [pz=deprotonated pyrazole, R= Me (3a), Et (3b), and tBu (3c)], R2Al(mu-3,5-Me2pz)2AIR2[3,5-Me2pz = deprotonated 3,5-dimethylpyrazole, R= Me (4a), and Et (4b)], and Et2B(mu-pz)2AlEt2 (5), has been investigated. We were particularly interested in the effect of steric hindrances both caused by the metal-bonded substituents and those that result from the nature of the bifunctional ligand used in the oxygenation reaction. In the reaction of 1 with O2, only the formation of the monoalkoxide compound6 was observed. The latter di-tert-butyl compound as well as all planar aluminapyrazoles, that is, the tert-butyl derivative 3c and lower alkylaluminum derivatives with the more demanding 3,5-dimethylpyrazoyl ligands 4a and 4b, are stable under an atmosphere of dry oxygen and ambient conditions. Inspection of the space-filling representation of these compounds has undoubtedly shown that the bulky tert-butyl groups or pyrazolyles ligands, respectively, provide steric protection for the metal center from the dioxygen attack. In contrast, the dialkylaluminum derivatives of pyrazole, 3a and 3b, and the diethylaluminum bis(1-pyrazolyl)borate complex 5, all with the metal center eclipsed with respect to the plane defined by the four nitrogen atoms, react smoothly with O2 to form the alkyl(alkoxy)aluminum complexes. In the reaction of 5 with O2 for example, the Et-B bonds remained intact, and the dimeric five-coordinate compound [Et2B(mu-pz)2 Al(mu-OEt)Et]2 (9) was isolated in good yield. The interaction of mononuclear di-tert-butyl chelate complex 2 with O2 at -15 degrees C gives (tBuOO)(tBuO)Al(mu-OtBu)2Al(mesal)2 (7) in high yield, and the presence of the alkylperoxo moiety is a particularly significant point in the resulting product. All the compounds have been characterized spectroscopically, and the structures of 3c, 4a, 6, 7, and 9 have been confirmed by X-ray crystallography. Structural features of 1-6 are discussed and are considered in relation to the possible approach pathways of the O2 molecule to the four-coordinate metal center. This analysis and the observed apparent dissimilarity in the reactions of model four-coordinate aluminum alkyls with O2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.

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Copper-mediated conjugate addition of allylic Grignard reagents to activated cyclopropane derivatives was studied. Unsaturated esters, malonates and phenylsulfonyl esters 2a-d were synthesized from the respective cyclopropanes 1a-d and methallylmagnesium chloride.Nucleophilic ring opening reactions involving activated cyclopropanes and amines, mercaptans, malonate anions or related Michael donors are well known. 1 Reaction of methallyllithium cuprate, which can be generated from isobutylene with the use of BuLi◊TMEDA complex, 2 with 1,1-dimethoxycarbonylcyclopropane provides the ring opened product 3 but requires a large excess of butyllithium which hampers its practical application. Addition of lithiumcuprate reagents to activated cyclopropanes have also been studied in detail in the context of certain natural product syntheses. 4,5 However, reaction of cyclopropane derivatives bearing two geminal alkoxycarbonyl or geminal alkoxycarbonyl and benzenesulfonyl groups with Grignard reagents has received little attention. Some time ago it has been shown that copper chloride-catalyzed addition of alkyl Grignard reagents to 1,1-dialkoxycarbonylcyclopropanes 6 and 1,1-diphenyl-sulfonylcyclopropane 7 occurs with opening of the cyclopropane ring. In the course of ongoing studies in one of our laboratories we were interested in applying activated cyclopropanes as a source of four or five carbon unit [CH 2 CH 2 CH 2 CO 2 R or CH 2 CH 2 CH(CO 2 R) 2 ] in the synthesis of higher aliphatic esters or malonates by a "homologous" Michael addition reaction involving Grignard reagents. For our work 1,1-dialkoxycarbonyl, 1-alkoxycarbonyl-1-phenylsulfonyl cyclopropane derivatives 1a-e (Scheme 1) and methallyl magnesium chloride were chosen.Treatment of di-tert-butyl ester 1a with methallylmagnesium chloride (2 mol equiv) and CuI◊Me 2 S (0.4 mol equiv) in THF at -30°C to -20°C afforded smoothly the 1,5-addition product 2a (Table). No traces of the respective 1,2-addition product 3a could be detected (a sample of 3a was prepared by reaction of 1a and the Grignard reagent with no CuI added).The reaction of the di-iso-propyl ester 1b with the Grignard reagent yielded a mixture of regioisomers 2b and 3b. The highest proportion of the required product (94:6) was attained when HMPA (1 mol equiv) was used as a cosolvent and the reaction was carried out at -30°C to -20°C for 5 hours and then at room temperature for 15 hours (Table, Entry 2). Eventually, pure compound 2b was obtained in 51% yield after chromatography. The reaction was slow at temperatures below -40°C (Table, Entry 3). On the other hand, at higher temperatures the degree of regioselectivity was lower (Table, Entry 4). Replacement of HMPA with DMPU 8 resulted in lowering of the regioselectivity of the reaction (Table, Entry 5). Scheme 1Downloaded by: University of Arizona Library. Copyrighted material.

The total diastereoselective synthesis of the C,D rings/side chain building block for the synthesis of 1α,25-dihydroxyvitamin D 3 is described. Two tandem Mukaiyama−Michael additions involving silylated ketene acetals derived from tert-butyl 6-methylhept-5-enethioate or tert-butyl 6-methylhept-6-enethioate, 2-methylcyclopent-2-en-1-one, and 1-(phenylthio)but-3-en-2-one afforded the corresponding intermediates with the complete carbon framework of the target compound. The further transformation of these key interme-