Chiral segment

Kitagawa 1, Tsujii S, Fujioka H, Kajiwara A, Yamamoto Y, Shibuya H, Chemical ttansfotmation of tetpenoids, VI, Syntheses of chiral segments, key building-blocks fot the left half of taxane-type ditetpenoids, Chem Pharm Bull 32 129 1302, 1984. 

Lipshutz, B. H. Kotsuki, H. Lew, W. En route to polyene macrolide total synthesis the key chiral segments of roflamycoin. Tetrahedron Lett. 1986, 27, 48254828. 

Figure 2-55. Dissection of planar achiral figures into achiral (a), hetcrochiral (b), and homo-chiral segments (c) some examples.

Kinoshita s erythronolide synthesis is based on the preparation of (9S)-9-dihydroerythronolide A (76), which is constructed from the three chiral segments, Ci-Cg (69), C7-C9 (70) and C10-C13 (72). 

In 1987, these authors presented a very productive strategy for constructing the chiral segments of the antibiotic by systematic functionalization of 4-substituted butyrolactones and butenolides, derived from a common chiral source, (S)-glutamic acid. 

As in Kinoshita s studies, all chiral segments have been prepared from proper derivatives of o-glucopyrano- or furanosides by judicious stereocontrolled transformations and hydroxy group manipulations in order to effect the required level of deoxygenation. 

CASA allows you to obtain R/S absolute stereochemistry, E/Z olefin geometry, Fischer projections, mirror images, and chiral segment recognition (D or L, DL, Meso). 

Deoxyribose was used as precursor for both chiral segments of (5S)-5-hydroxy-l4,15-LTA (56), the biogenetic precursor of the lipoxins (sugar numbers indicated), and the carbons of D-xylose were incorporated as indicated into the salt marsh caterpillar moth... 

Identify segments of nCL stereocenters which in some degree map onto available chiral predecessors in order to generate a chiral S-goal. 

Fig. 6. Model structure of the (8,1H6.4) knee extended by two straight chiral tubule segments.

Ti in [Ti(> 2-Cl04)4] and Ni" in [Ni(>j -C104)L2] where L is a chiral bidenlate organic ligand.Sometimes both and modes occur in the same compound. The biden-latc bridging mode occurs in the silver complex [Ag /x,>j -00(0)20- (m-xylene)2]- The structure of appropriate segments of some of these compounds arc in Fig. 17.23. The distinction between coordinated and non-coordinaied ( ionic ) perchlorate is sometimes hard to make and there is an almost continuous... 

This methodology has been used for the synthesis of the C3-C14 segment 24 of the antitumor agent laulimalide 23 (Scheme 4.22) [35]. The constrained chiral BOX ligand 21c in combination with Cu(OTf)2 afforded dihydropyrane 6f by a cycloaddition reaction in good yield and ee this was converted to the C3-C14 segment 24 via a Ferrier-type rearrangement in several steps. 

A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. 

The first (+)-discodermolide synthesis was completed by Stuart Schreiber s group at Harvard University and is outlined in Scheme 13.68. This synthesis was carried through for both enantiomers and established the absolute configuration of the natural material. The retrosynthetic plan outlined in Scheme 13.67 emphasizes the stereochemical triads found at C(2)-C(4), C(10)-C(12) and C(18)-C(20) and was designed to use a common chiral starting material. Each of the segments contains one of the stereochemical triads. 

The C(9)-C(14) segment VI was prepared by Steps D-l to D-3. The formation of the vinyl iodide in Step D-3 was difficult and proceeded in only 25-30% yield. The C(15)-C(21) segment VII was synthesized from the common intermediate 17 by Steps E-l to E-6. A DDQ oxidation led to formation of a 1,3-dioxane ring in Step E-l. The A-methoxy amide was converted to an aldehyde by LiAlH4 reduction and the chain was extended to include C(14) and C(15) using a boron enolate of an oxazo-lidinone chiral auxiliary. After reductive removal of the chiral auxiliary, the primary alcohol group was converted to a primary iodide. The overall yield for these steps was about 25%. 

Macrocyclic stereoisomeric phosphoramidates 80 and 81 containing oxadiazole segments were obtained starting from 2,5-(2 -hydroxyphenyl)-l,3,4-oxadiazole, phosphorus oxychloride, and other reagents <2001JST145>. Chiral macrocyclic phosphoramidates were also obtained <2001SC3197>. 

Figure 4.35 shows the CD and UV spectra for 48 in isooctane. The observation of the Cotton effect indicates that the chromophore in the polymer, which is a main-chain segment over which the silicon a and o orbitals are electronically delocalized, exists in a chiral structure a helix. 

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