Backbone structures stereochemistry

The term diversity-oriented synthesis (DOS) is relatively new and, as mentioned above, is usually defined as the synthesis of complex, natural product-like molecules using a combinatorial approach and employing the full palette of modern organic reactions. It may be a subject of discussion what exactly qualifies a molecule as being natural product-like [4], and in most cases the similarity to an actual natural product seems reciprocal to the number of synthesized compounds. However, even in less complex cases, the products may be highly substituted polycyclic structures with defined stereochemistry, reminiscent of natural products [19, 20]. In these cases, a moderately complex backbone structure is subsequently modified with a well-established set of selective reactions to introduce diversity. 

Intramolecular cycloaddition of fV-benzyl-substituted 3-O-allylhexose nitrones furnishes chiral oxepane derivatives. The regioselectivity of the cycloaddition depends on several factors such as (1) the structural nature of the nitrone, (2) substitution and stereochemistry at 3-C of the carbohydrate backbone, and (3) substitution at the terminus of the O-allyl moiety. A mixture of an oxepane and a pyran is formed in the intramolecular oxime olefin cycloaddition of a 3-O-allyl carbohydrate-derived oxime <2003T4623>. The highly stereoselective synthesis of oxepanes proceeds by intramolecular nitrone cycloaddition reactions on sugar-derived methallyl ethers <2003TA3899>. 

NMR analysis of. sm>-D-ring taxane analogues <1999BML3041, 2000JNP726> supports the hypothesis that the oxetane serves to rigidify the overall molecular backbone (see Section 2.06.12.3). NOE and nuclear Overhauser effect spectroscopy (NOESY) experiments have been used to establish stereochemistry in taxanes, their synthetic precursors, and model structures <2005JOC3484, 2001S1013>. Fluorescence spectroscopy and rotational-echo double... 

Since Hirata et al. began research into daphniphyllum alkaloids in 1966, a number of new alkaloids have been discovered. As a result, the number of known daphniphyllum alkaloids has grown markedly in recent years to a present count of 118 (compounds 1-118). These alkaloids, isolated chiefly by Yamamura and Hirata et al. are classified into six different types of backbone skeletons [1-3]. These unusual ring systems have attracted great interest as challenging targets for total synthesis or biosynthetic studies. This chapter covers the reports on daphniphyllum alkaloids that have been published between 1966 and 2006. Since the structures and stereochemistry of these alkaloids are quite complex and the representation of the structure formula has not been unified, all the natural daphniphyllum alkaloids (1-118) are listed. Classification of the alkaloids basically follows that of the previous reviews [1,2], but sections on the newly found skeletons have been added. 

Subsequent reports by Fleet and co-workers showed that the occurrence and specific type of secondary structure in these furanose-based carbopeptoid foldamers was strongly dependent on both the backbone stereochemistry and the stereochemistry of the ring substituents (Figure 51).355 351 Various ris-linked furanoses mostly exhibit the repeating /3-turn-like conformation mentioned above.360 361 However, one of the cis-linked stereoisomers displayed no indication of secondary structure.359 This surprising result was rationalized in that one methyl group in the isopro-pylidene unit of the fused ring is positioned in a way... 

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