Retention of the configuration

The photochemistry of cyclobutanones differs from that of less strained larger cycloalkanones. Fragmentation to ethylene and ketene (derivatives), decarbonylation and rearrangement to oxacarbenes predominate here. The oxacarbene formation, which occurs with retention of the configuration of the... 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

In a similar way, 5-O-acetylthymidine was converted into the 3-deoxy-3-iodo derivative 72 in 55% yield. In this case, the replacement of the hydroxyl group by iodine was presumed to have taken place by retention of the configuration at C-3. The first intermediate in the reaction was proposed to be the phosphonate (70) which rapidly collapses to an O-3-cyclonucleoside (71) and the latter is subsequently attacked by iodide ion to give the product 72. It was also observed (106) that treatment of nucleosides containing a cis vicinal diol grouping such as 5-0-acetyluridine with triphenylphosphite methiodide failed to provide iodinated products but gave phosphonate derivatives instead. 

The all-d.v benzamide 12, readily obtainable from 8 at low temperature with methyllithium,16 is cleanly transformed to the mono-trans isomer 13 above O C. Both anions 12, 13 can be trapped at low temperature with methyl chloroformate to yield the corresponding methyl azepinc-A -carboxylates 14 and 15, respectively, with retention of the configuration.17... 

Recently, Uneyama reported that treatment of (R)-l-tosyl-2-trifTuoromethylazir-idine 76 (Scheme 3.24) with w-BuLi at -100 °C and subsequent trapping of the anion with electrophiles such as chloroformates produced aziridine-2-carboxylates 77 in good to excellent yields [71]. The retention of the configuration of the tri-fluoromethylated quaternary carbon center in the course of the reaction was confirmed by derivatization of the product and by X-ray studies. 

Short-lived chiral ion pairs are intermediates in the Haller-Bauer cleavage 14 15 of enantiomer-ically enriched 2,2-disubstituted 1,2-diphenylethanones, which give optically active phenylalka-nes on in situ protonation with partial retention of the configuration. 

Both glucosyl copper reagents / -6c and a-8 underwent conjugate addition with complete retention of the configuration of the anomeric carbon and neither product isomer was contaminated with detectable levels of the other. 

Photochemical reactions of carbonyl compounds with alkenes give the oxetanes (Scheme 30). The stereochanical course depends on the substituents of the alkenes [16]. The reactions proceed with the retention of the configuration of the alkenes for the electron accepting substituent, e.g., CN. The stereochemical integrity is lost for the donating group, e.g., OCH. ... 

Cycloaddition reactions of ketenes with alkenes have long been known to give cyclobutanones [123] and to proceed with retention of the configurations [124], The reactions were classified into the symmetry-allowed cycloaddition reactions... 

Similar additions also occurred on vinylphosphine oxides. When the optically active vinylphosphine oxide was used, P-chiral alkylphosphine oxide was obtained with retention of the configuration (Eq. 10.27)60... 

Phthalideisoquinoline alkaloids have been suggested to be biosynthesized from 13-oxyberbines with retention of the configuration at C-13 and C-14 through regioselective C-8—N bond cleavage (185). Based on this biogenetic viewpoint, ( )-ophiocarpine and ( )-epiophiocarpine were converted to ( + )-a- and ( )-/ -hydrastine, respectively (Scheme 70) (186). Treatment of... 

The hydrogenolysis of the C-O bond proceeds mainly with inversion of the configuration of the carbon atom over a Pd catalyst but with retention of the configuration on Ni (Scheme 4.4).3-6... 

They react with a wide range of aliphatic and aromatic aldehydes in the presence of catalytic amounts of tetrabutylammonium fluoride (TBAF) to give the trialkylsilyl ethers of P-nitro alcohols with high anti-selectivity (98%). The diastereoselective Henry reaction is summarized in Table 3.2. The products are reduced to P-amino alcohols using Raney Ni-H2 with retention of the configuration of P-nitro alcohols (Scheme 3.12). 

Ono and Kamimura have found a very simple method for the stereo-control of the Michael addition of thiols, selenols, or alcohols. The Michael addition of thiolate anions to nitroalkenes followed by protonation at -78 °C gives anti-(J-nitro sulfides (Eq. 4.8).11 This procedure can be extended to the preparation of a/jti-(3-nitro selenides (Eq. 4.9)12 and a/jti-(3-nitro ethers (Eq. 4.10).13 The addition products of benzyl alcohol are converted into P-amino alcohols with the retention of the configuration, which is a useful method for anri-P-amino alcohols. This is an alternative method of stereoselective nitro-aldol reactions (Section 3.3). The anti selectivity of these reactions is explained on the basis of stereoselective protonation to nitronate anion intermediates. The high stereoselectivity requires heteroatom substituents on the P-position of the nitro group. The computational calculation exhibits that the heteroatom covers one site of the plane of the nitronate anion.14... 

Amino alcohols like (iS )-prolinol react with nitroalkenes very rapidly with very high facia] selectivity.31 Rapid and stereoselective reduction of the nitro function is essential for the conversion of the products to 1,2-diamine derivatives with the retention of the configuration. Samarium diiodide is recommended in the stereoselective reduction of thermally unstable 2-aminonitroalkanes to give a range of useful 1,2-diamines (Eq. 4.26).32... 

X-ray diffraction studies of the Co(n) and Co(m) sepulchrate complexes indicate that they have similar structures although, as expected, the M-N bond lengths are shorter (1.99 A) in the Co(m) complex than in the Co(ii) species (2.16 A). The Co(n) complex was obtained by direct reduction of the Co(m) analogue using zinc dust. A striking feature of the syntheses of these species is that the reactions proceed with retention of the configuration of the [Co(l,2-diaminoethane)3]3+ starting complex. 

Deamination of 2-amino-2-deoxyaldonic acids with nitrous acid has been shown to give 2,5-anhydroaldonic acids with overall retention of the configuration of C-2. The reaction parallels the deamination, with nitrous acid, of a-amino acids that leads to a-hydroxy acids with retention of configuration.19,29 Thus, the formation of 2,5-anhydro-D-gluconic acid27 (15) from 2-amino-2-deoxy-D-gluconic acid (14) has... 

Deoxygenation of Cyclic Nitronates Since cyclic nitronates are readily available and are rather stable compounds (see Sections 3.2 and 3.3), simple approaches for their deoxygenation with retention of the configuration of stereocenters can be used for the synthesis of cyclic esters of functionalized oximes. 

Selective oxidation of allylic alcohols.1 This zircononcene complex when used in catalytic amount can effect an Oppenauer-type oxidation of alcohols, including allylic ones, in the presence of a hydrogen acceptor, usually benzaldehyde or cyclohexanone. This system oxidizes primary alcohols selectively in the presence of secondary ones. Thus primary allylic alcohols are oxidized to the enals with retention of the configuration of the double bond in 75-95% yield. The method is not useful for oxidation of propargylic alcohols. 

A G bond is considered to be involved in a cycloaddition in a supra manner if configuration is either retained or inverted at both of its termini in the course of the reaction. The retention of the configuration at one terminus and the inversion at the other will indicate an antara addition. 

Substrates with carbamate-protected [81, 82] and even free hydroxyl groups [69] reacted similarly in a deprotonation-reprotonation sequence, the latter even with retention of the -configuration of an alkene such as 46 (Scheme 1.19). The analogous (E)-alkene also delivers only E-product. 

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