Retrosynthetic fragments

Peripheral structural motifs are a second principal category of retrosynthetic fragments. The presence of such fragments usually does not substantially influence... 

Peripheral Retrosynthetic Fragments Flow to Measure the Target-specific Differences. ... 

A self-organizing Kohonen map of the total database of cleaved retrosynthetic fragments generated as the result of an unsupervised learning procedure (data not shown)indicates that the cleaved fragments occupy a wide area on the map, characterized... 

Two formal total syntheses of calcimycin have been achieved (94-96). They are similar in concept in that the retrosynthetic analyses entail disconnection of the l,7-dioxaspiro[5.5]undecane moiety of 168 to the ketodiol precursor 170 which would readily yield calcimycin on acid-catalyzed cyclization (spiro-ketalization). Further retrosynthetic fragmentation of ketodiol 170 into the pyr-... 

We begin our discussion of the retrosynthetic fragments with the thiazole moiety A (cf. Scheme 1), i.e. the least problematic building block. Indeed, they appear to be so easily accessible that some authors do not mention their synthesis or references, and sometimes even forget to count the necessary steps in their final eonelusion. 

Figure 10.3-30. The retrosynthetic point of view the transform of a Michael addition. The structure fragment with a gray background is the retron of the Michael addition transforin.

A symbol used to indicate a retrosynthetic step is an open arrow written from prod uct to suitable precursors or fragments of those precursors... 

The antibiotic aplasmomycin serves as a receptor or transporter for borate. Retrosynthetic simplification of the boron-free macrocycle to identical hydroxy acid subunits is clearly appropriate. Further retrosynthetic dissection produced fragments A, B and oxalate and provided a workable synthetic plan. 

A retrosynthetic analysis of fragment 152 can be completed through cleavage of the C16-C17 bond in enone 155, the projected precursor of epoxide 152. This retrosynthetic maneuver furnishes intermediates 156 and 157 as potential building blocks. In the forward sense, acylation of a vinyl metal species derived from 156 with Weinreb amide 157 could accomplish the construction of enone 155. Iodide 153, on the other hand, can be traced retrosynthetically to the commercially available, optically active building block methyl (S)-(+)-3-hydroxy-2-methyIpropionate (154). 

We now tum our attention to the C21-C28 fragment 158. Our retrosynthetic analysis of 158 (see Scheme 42) identifies an expedient synthetic pathway that features the union of two chiral pool derived building blocks (161+162) through an Evans asymmetric aldol reaction. Aldehyde 162, the projected electrophile for the aldol reaction, can be crafted in enantiomerically pure form from commercially available 1,3,4,6-di-O-benzylidene-D-mannitol (183) (see Scheme 45). As anticipated, the two free hydroxyls in the latter substance are methylated smoothly upon exposure to several equivalents each of sodium hydride and methyl iodide. Tetraol 184 can then be revealed after hydrogenolysis of both benzylidene acetals. With four free hydroxyl groups, compound 184 could conceivably present differentiation problems nevertheless, it is possible to selectively protect the two primary hydroxyl groups in 184 in... 

Lewell XQ, Judd DB, Watson SP, Hann MM. RECAP—retrosynthetic combinatorial analysis procedure a powerful new techniqne for identifying privileged molecular fragments with useful applications in combinatorial chemistry. J Chem Inf Comput Sci 1998 38 511-22. 

The first diastereoselective syntheses of juvabione are described in Schemes 13.11 and 13.12. Scheme 13.10 is a retrosynthetic analysis corresponding to these syntheses, which have certain similarities. Both syntheses started with cyclohexenone, there is a general similarity in the fragments that were utilized, although the order of construction differs, and both led to ( )-juvabione. 

Scheme 13.67. Retrosynthetic Analysis of (+)-Discodermolide to Fragments containing Stereotriads3... 

---