Branched carbon chains scheme

A multi-step reaction sequence was then realized to prepare the precursor (178) for the pivotal macrocyclization reaction. Alternate stepwise chain elongations were achieved according to Schemes 28 and 29. Reaction of the tosylate prepared from the alcohol 162 with lithium acetylide afforded the alkyne 174 (Scheme 28). Following the introduction of a tosylate at the upper branch, a one-carbon chain elongation of the terminal alkyne afforded the methyl alkynoate 175. A methyl cuprate 1,4-addition was used to construct the tri-substituted C double bond stereoselectively. For this purpose, the alkynoate 175 was initially transformed into the Z-configured a,/ -unsat-... 

We thus investigated a range of such one-carbon electrophiles (Scheme 10). Formaldehyd, as the most reactive one, gave the desired product directly (28). Using benzaldehyde as the electrophile likewise lead to a C-2 branched aldonolactone in a high yield and stereospecificity. The two diastereoisomeric compounds obtained, due to the new chiral center formed in the side chain, were both isolated in a crystalline state (28). 

Most of the methods presented for the de novo synthesis of linear monosaccharides can be used to prepare branched-chain sugars and analogs and some examples are given above and below. The branched-chain aminolactone 222 was prepared by two-carbon chain elongation via addition of 221 to 2,3-0-isopropylidene-L-glyceraldehyde (Scheme 13.69) [126]. 

Fig. 7. Scheme of carbon chain growth to form linear alcohols, branched alcohols, and methyl esters over alkali-promoted Cu/ZnO-based catalysts. 

Biosynthetic studies feeding [l- c]- and [l,2-13c2]-acetate determined that 79 contained a tetraketide unit (Scheme 1). [Me-13C]-methionine supplied the branched carbon on the side chain and the two methoxy carbons. The origin of the remainder of the pyridone ring has yet to be established [218],... 

Scheme 2 How to Generate Chiral Methyl Branching in a Carbon Chain to Form an 1,2-Hydroxy-Methyl (1,2-HM) Subunit...

One of the best of these schemes (10, II), SCG (simple chain growth), involved one-carbon addition to one end of the growing chain at the first carbon with a probability a and to the second carbon with a probability af, if addition had not already occurred on this carbon. The SCG scheme assumed that the growth constants were independent of carbon number and structure of the growing chain, a situation that is fortuitous rather than expected. For a given carbon number, the ratios of branched to normal hydrocarbons are f or 2f for monoethyl isomers and P or 2P for dimethyls the factor one or two depends on whether the species can be produced in one or two ways. This simple mechanism predicted carbon number and isomer distributions moderately well and also seemed consistent with the tagged alcohol incorporation studies of Emmett, Kummer, and Hall (12,13,14). SCG does not produce ethyl-substituted carbon chains, which were subsequently found to be present in about the same concentration as dimethyl species (15,16). 

The third isomer in Scheme 4.4c uses a C fragment, thus creating a six-carbon chain with two branches. Again, the C fragment can be placed in different locations along the chain, and one can thereby create more isomers. 

Two sets of branched-chain sugar phosphonates, having the phosphonate attached to the C-3 branch carbon, have been synthesized, one such set being Illustrated in Scheme 3, and a variety of compounds of this type and related phosphinates and phosphine oxides... 

The appropriately substituted azido-carbohydrate derivatives (48) have been shown to undergo a (3,3)-sigmatropic rearran ment, leading to the thermodynamically more stable 2-ethyleneazides (49) that possess an endocyclic double bond (Scheme 58). The rearrangement thus transposes the double bond into the ring at the branch carbon and the azido-group into the side chain. 

Lactone 8.17 is a 1,5-difunctional molecule at a lower oxidation state than 8.16 and is accessible by the same discoimection rationale (Scheme 8.12). First disconnect the functionality and then the carbon chain at the branch point (C3-C4 bond). The butyraldehyde enolate called for here is not practical because of self-condensation (Section 7.1.1) therefore, a nonelectrophilic derivative of the aldehyde is used instead. 

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