C-acetyl-branched

The structural identification of the sugar from the B components as a C-acetyl-branched sugar constituted strong support for the conclusion reached from work on the biosynthesis of D-aldgarose (see Section II,2,a) that a C-acetyl-branched sugar is the primary product in the attachment of the two-carbon branch (see Scheme 6). 

The incorporation of [2-14C]pyruvate and [l-14C]acetate into sugars 17 and 18 was investigated.27 Oxidation of the methyl glycosides of sugar 17 with periodate yielded acetaldehyde from the 1-hydroxyethyl branch. The acetaldehyde (2,4-dinitrophenyl)hydrazone was further oxidized by Kuhn-Roth oxidation to acetic acid, which was degraded by the Schmidt reaction to methylamine and carbon dioxide. Periodate oxidation of the methyl glycosides of sugar 18 produced acetic acid from the C-acetyl branch. The acetic acid was isolated, and purified as 1-acetamidonaphthalene. 

The following results were obtained with these degradation reactions. [2-14C]Pyruvate was specifically incorporated into the 1-hydroxyethyl branch of 17 and into the C-acetyl branch of 18. In each instance, —90% of the radioactivity of the methyl glycosides was... 

On the basis of the results discussed and of studies on the time-dependence of formation of product from dTDP-D-glucose, the sequence of reactions shown in Scheme 7 was postulated28 for the biosynthesis of the C-acetyl-branched sugar 19. It is at present unknown why the sugar formed in the cell-free system is the C-3 epimer of sugar 18 from quinocycline B. 

The configuration at the C-methyl branch was deduced from the position of the acetamido resonances in the tetraacetates of (54)—(57), indicating equatorial orientations of this group (NHAc at t = 8,15, 8,16 and 8,19 in CDCls) for the acetates of (54)—(56), in contrast to the acetyl derivative of (57), which exhibited no signal above t = 7,95, (axial NHAc) (see Sect. 2.8)... 

Table. Chemical shifts in CDCI3 of substituent resonances in fully acetylated C-methyl branched aminocyclanols and aminosugars...

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

Fig. 1. Synthesis of ring-fused and C-2 branched 1,5-anhydroalditols.125 Conditions A, addition of n-BujSnH during 12 h B, n-Bu3SnCl (0.1 eq) and NaBH3CN (2 eq) C, radical cyclization followed by treatment with KF, oxidation, and acetylation.

The stepwise synthesis of methyl 4-(9-[3,4-di-0-( 3-D-xylopyranosyl)-j3-D-xylopyranosyl]-/3-D-xylopyranoside (17), a methyl xylotetraoside related to branched xylans, has been achieved by stereoselective condensation of 2,4-di-(9-acetyl-2-O-benzyl-a-D-xylopyranosyl bromide with methyl 2,3-anhydro- -D-ribopyranoside and reaction of the deacetylated product with 2,3,4-tri-C>-acetyl-a-D-xylopyranosyl bromide. 

In a radical-based method for preparing C-2 branched-chain sugars by way of addition of dimethyl malonate to tri-O-acetyl-o-glucal (see Chapter 14), 1,4,6-tri-0-acetyl-2,3-dideoxy-a,p-D-erytAro-hex-2-enopyranose was formed as a byproduct in a small amount. 

When allowed to react with sodium acetate in 2-methoxyethanol-water (9 1), de-O-mesylation takes place to yield a product (60), identical in all respects with the compound prepared from the waMMO-derivative (56) via N-acetylation (to (67)) and benzylidenation. However, since de-O-mesylation in (59) can occur either with participation of the nucleobase via the anhydronucleoside (62), or with participation of the acetamido group through the oxazoline (63), the steric relationship between C-2 and the branch point cannot be deduced unequivocally. Yet, when modifying the de-O-mesylation conditions to sodium methoxide/2-... 

A branched-chain iodo sugar derivative, l,5-anhydro-4,6-0-benzyl-idene-2,3-dideoxy-3-C-(iodomethyl)-D-rifoo-hex-l-enitol [4,6-O-ben-zylidene-3-deoxy-3-C-(iodomethyl)-D-allal] (200), is one of the products formed on treatment of methyl 4,6-0-benzylidene-2,3-dideoxy-a-D-en/thro-hex-2-enopyranoside (77) with the Simmons-Smith reagent (diiodomethane and zinc-copper couple).123,212 Compound 200 displays high solvolytic reactivity, an observation that has been rationalized by supposing the formation of the highly stabilized carbonium ion213 (201). Thus, under conditions wherein methyl 2,3,4-tri-0-acetyl-6-deoxy-6-iodo-a-D-glucopyranoside required more than 24 hours to react appreciably with an excess of silver nitrate in 50% aqueous p-dioxane buffered with silver carbonate, the iodide 200 was hydrolyzed completely in less than 1 minute the product of hydrolysis of 200 is the cyclopropyl aldehyde 202. Methanolysis of... 

The synthesis of all isoprenoids starts with acetyl-CoA, which in a series of six different enzyme reactions is converted into isopentenyl-diphosphate (-PP), the basic C-5 isoprene unit that is used for the synthesis of all subsequent isoprenoids (Fig. 5.1.1). At the level of farnesyl-PP the pathway divides into several branches that are involved in the production of the various isoprenoid end products. One of the major branches involves the cholesterol biosynthetic part of the pathway, of which squalene is the first committed intermediate in the production of sterols. Following cycliza-tion of squalene, lanosterol is produced. To eventually produce cholesterol from la-... 

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