Branched-chain stereochemistry

In most cases, the stereochemistry of the quaternary carbon atom in branched-chain carbohydrates cannot be elucidated from H-n.m.r. spectra, but 13C-chemical shifts, or long-range, 13C- H coupling-constants, may often yield valuable information.114-118 Likewise, the stereochemistry of acetal carbon atoms of benzylidene derivatives,103, 104 and of acetals derived from pyruvic acid,105,106 may be determined from l3C-chemical shifts. 

Yunker, M B, Plaumann, D E, Fraser-Reid, B, The stereochemistry of conjugate addition of lithium dialkyl cuprate reagents to some carbohydrate a-enones. Can. J. Chem., 55, 4002-4009, 1977. Baer, H H, Ong, K S, Raeactions of nitro sugars. IX. The synthesis of branched-chain dinitro sugars by Michael addition. Can. J. Chem., 46, 2511-2517, 1968. 

Comparing the strategies from Schemes 11.16 and 11.17 illustrates that fine tuning the starting compound allows one to choose the stereochemistry at C2 of the branched chain. It is clear that this method compares well with other 2-C-methylation processes in terms of length and stereoselectivity, and should find many applications in the field. 

The branched-chain amino acids valine, leucine, and isoleucine have very similar biosyntheses and catabolism. Valine 179 and leucine 205 have diastereotopic methyl groups and leucine 205 and isoleucine 212 have diastereotopic hydrogen atoms. These may be differentiated by labeling so that the stereochemistry of the biological processes involving these amino acids may be studied. 

The biosynthesis of the macrolactam polyketide glycoside anti-tumour antibiotic vicenistatin L (1), including its vicenisamine moiety, has been studied using heavy atom-labelled precursors. The bicyclic branched-chain diaminohep-turonic acid derivative 2, a tyrosyl tRNA synthetase inhibitor of unknown absolute stereochemistry, has been isolated from a Microsmonospora species fermentation medium. ... 

The structures and stereochemistry of citreodiol (99)(sugar numbering) and epicitreodiol, metabolites of Penicillium citreo-viride. have been determined by synthesis of their antipodes from L-rhamnose two routes led to the epimeric branched-chain glycenonic esters (100), which were converted to (99) and its epimer by a conventional Wittig sequence.An intramolecular Diels-Alder... 

An orthoester derived from the exocyclic unsaturated branched-chain sugar (41) underwent Claisen rearrangement to form the gem-disubstituted branched-chain sugar (42)(Scheme 14) the stereochemistry of (42) was deduced from the fact that the derived C-formyl sugar (43) did not form a hemiacetal. The 5-epimer of... 

Introduction of C-3 branch point functionalities that can be later deblocked and/or oxidized to form a carboxylic group is possible via at least two different routes (Figure S). One commonly used approach in the synthesis of branched-chain sugars involves addition of a nucleophile to an ulose (25). However, use of 2-dithianyllithium (2<5), vinyl magnesium bromide (27) or 2-f l lidiium (25) is not likely to lead to the correct C-3 stereochemistry since the preferred direction of their approach to ulose 7 will be from the sterically less demanding eco-face... 

Most pyrrolizidine alkaloids are esters constructed from an aminoalco-hol called a necine unit (pyrrolizidine derivative), and straight or branched chain fatty acid, called a necic acid. Necine is the derivative of pyrrolizidine, i.e., azabicyclo[3,3,0]octane, and a hydroxymethyl (-CH2OH) moiety is always attached at the C-1 position. Necines can be classified into two groups, either with or without a double bond between the C-1 and C-2 positions. The stereochemistry of the H-8 position is usually H-8a.When the C-8 position is oxidized, as in the case of otonecine, it becomes an eight-membered ring by decychzation. 

A new branched-chain nonitol, calditol, assigned the structure (1), has been isolated as a component of the lipids in Calderella thermoacidophile bacteria, in which it provides the hydrophilic tail the stereochemistry was not determined. The effects of pH, solvent, and catalyst treatment on the Raney nickel-catalysed hydrogenation of xylose to xylitol has been studied, and the factors governing this process have been reviewed. The yields of D-mannitol obtained by... 

Tri-C -benzyl-D-/nyo-inositol has been achieved from a protected a-D- i /o-hex-5-enopyranoside by way of a Perrier reaction. In a similar way the branched-chain sugar 95 was used as starting material for the synthesis of (-)-laminitol and mytilitol." On the mechanism of the Ferrier reaction, it has been observed that there is loss of stereochemistry at C-6 in the deuterated eno-pyranoside 96 during the formation of the carbocyclic ring." ... 

Stereodivergent pathways relative to other allylic systems (e.g. allylic acetates) have been found in the reactions of cyclic allylic sulfides or ethers of benzothiazole with organocopper reagents. In this case the Sn2 process occurs with syn stereochemistry (eq 5) and is due to the anchimeric coordination of the organometallic reagent with the heterocyclic moiety. This has been utilized for regio- and stereocontroUed access to branched-chain sugars. 

Carbohydrate a-nitroepoxides react with methylmagnesium iodide to yield branched-chain amino-sugars as outlined in Scheme 8, but the stereochemistry of the process has not been elucidated. ... 

In general, alkenes with branched chains are more stable than unbranched isomers. This effect is usually less important than the degree of substitution or stereochemistry of the double bond. 

Dianhydro-D-galactopyranose has been used as a precursor for the preparation of the double-branched-chain sugar (34) by the sequence traced in Scheme 10. A related sequence likewise yielded the novel dianhydro-branched-chain sugars (35), the stereochemistry of epoxidation,using the Sharpless reagent,depending on whether... 

The stereochemistry of the products of nucleophilic addition of phenylacetic acid dianion to l,2 5,6-di-Q-cyclohexylidene-a-D-ti -hexofuranos-3-ulose has been determined - partly by use of circular dichroism. Base treatment of the Q-mesylcyanohydrin (1) of the furanos-3-ulose (2) afforded spiro-oxathiole-2,2-dioxide (3) which cleaved with methoxide anion to give the branched-chain sugar derivative (4) (Scheme 1). Similar results were obtained... 

It has been established that the kinetically favoured product formed on the addition of cyanide ion to l,2 5,6-di-0-isopropylidene-a-D-r/6o-hexofuranos-3-ulose has the D-a//o-configuration, whereas the thermodynamically favoured product has the D- / co-configuration. Conversion of the D-a/to-isomer into the lactone derivative (248) was used to establish its stereochemistry. Other branched-chain derivatives, including the amino-sugar (249), were also prepared. 

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