C„ fragment

Table 9. Alkaloids Derived from Tyrosine as a Ce-C, Fragment and Phenylalanine as a Ce-C3 Fragment...

Introduction of Nitrogen into a Terpenoid Skeleton. The acetate-derived fragments (35) mevalonic acid (30), which yields isopentenyl pyrophosphate (31) and its isomer, 3,3-dimethyl ally pyrophosphate (32) a dimeric C -fragment, geranyl pyrophosphate (33), which gives rise to the iridoid loganin (34) and the trimer famesyl pyrophosphate (35), which is also considered the precursor to C q steroids, have already been mentioned (see Table 3... 

At the time of the discovery of Met-enkephalin, its sequence was observed to be identical to that of residues 61—65 contained in the C-fragment of the pituitary hormone p-Hpotropin [12584-99-5] (p-LPH) (see Hormones), first isolated in 1964 (11). In 1976, the isolation of a larger peptide fragment, P-endorphin [60617-12-1] that also displayed opiate-like activity was reported (12). This peptide s 31-amino-acid sequence comprised residues 61—91 of P-LPH. Subsequentiy, another potent opioid peptide, dynorphin [72957-38-17, was isolated from pituitary (13). The first five amino acids (qv) of this 17-amino-acid peptide are identical to the Leu-enkephalin sequence (see Table 1). 

The core of the first synthesis of vitamin involved condensation of the A—D ting fragment (10) with the B—C fragment (11). The former... 

The keto-carbonyl C signal at 5c = 200.9 would only fit the aflatoxins B, and M,. In the C NMR spectrum an enol ether-C// fragment can also be recognised from the chemical shift value of 5c = 145.8 and the typical one-bond coupling constant Jch = 196 Hz the proton involved appears at Sh = 72, as the CH COSY plot shows. The H triplet which belongs to it overlaps with a sing-... 

In 1972, van Leusen, Hoogenboom and Siderius introduced the utility of TosMIC for the synthesis of azoles (pyrroles, oxazoles, imidazoles, thiazoles, etc.) by delivering a C-N-C fragment to polarized double bonds. In addition to the synthesis of 5-phenyloxazole, they also described reaction of TosMIC with /7-nitro- and /7-chloro-benzaldehyde (3) to provide analogous oxazoles 4 in 91% and 57% yield, respectively. Reaction of TosMIC with acid chlorides, anhydrides, or esters leads to oxazoles in which the tosyl group is retained. For example, reaction of acetic anhydride and TosMIC furnish oxazole 5 in 73% yield. ... 

Scheme 27 Trimethyl phosphite as alkoxyl radical scavenger to characterize C-0 vs C-C fragmentation for oxyranylcarbinyl radicals. Reprinted with permission from [64]. Copyright 2003 American Chemical Society...

Many 5-alkyl-, 5-alkenyl-, and 5-alkynyl-l-(2-deoxy-2-fluoro- 5-D-arabin-ofuranosyl)pyrimidines (750 - 752 and 765 - 767) were prepared - " by condensation of 742 (or the equivalent, such as 743) with C-5-substituted pyrimidine derivatives, or by attachment of a C-fragment to C-5 of the existing nucleosides through their 5-iodo or 5-chloromercuri derivatives, or by further conversion of the attached C-5 substituent into another one. ... 

Fig. 24.7 The conversion of preproinsulin to insulin by sequential removal of the signal peptide and the C fragment.

We do not refer to elements heavier than Se except for Mo and W since the others are not used in biological catalysts. Note the different use of early transition metals in O transfer (and N transfer), while later transition metals are used in H- and C-fragment transfer (see Section 2.20). 

Both approaches include the reaction of BENA with ternary amines or the -C=N-C-fragment of a nitrogen heterocycle (Nu) in the presence of a silylating agent (Me3SiX). 

Alternatively, the ring C/D enlargement-contraction to form the spiroin-dolenine moiety could take place via an electrophilic attack on bisindolenamine 323 to afford the iminium ion 324 or its equivalent carbinolamine 325. Ring closure to hydroxyaziridinium ion 326, followed by C—C fragmentation could... 

Fig. 15. Sigma fluorine lone pair interactions with the sigma MO s of the C=C fragment...

Similarly, the combination of three different types of fragments, namely, A, B and two C fragments to yield AB plus C2 may be more or less stable than the combination of the same fragments to yield AC plus BC depending upon the donor-acceptor interrelationships. In all cases, however, the success of the approach is guaranteed only if matrix elements remain relatively constant. 

The oxypalladation method mentioned above was introduced as a crucial step in the synthesis of several natural products. As shown in Scheme 8.51, Metz and coworkers used this strategy in an enantioselective synthesis of ricciocarpin A [122], Other impressive applications including the acetalization-RCM sequence have been employed in the synthesis of the AB ring of ciguatoxin [123] and of the Q-C fragment of laulimalide [124] (Scheme 8.52). 

Ottinger, C. Fragmentation Energies of Metastable Organic Ions. Phys. Lett. 1965, 17,269-271. 

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