Natural products donors

The cyclopropylimine ynoline rearrangement (equation 68) has been exploited by Stevens in alkaloid syntheses. - Wenkert s cyclopropylcarbinyl rearrangement (equation 69) served extremely well in the design of 1,4-dicarbonyl synthons or 3,-y-unsaturated carbonyl compounds which then were expressed in numerous syntheses of terpenoid and alkaloid natural products. Donor-acceptor concepts continue to be express in the applicability of these rearrangements to organic synthesis. 

Tridachiahydropyrone belongs to the family of marine polypropionates [69]. Efforts towards its total synthesis have recently led to a revision of the structure with the new proposal 2-147 [70]. The construction of the highly substituted cyclohex-enone moiety 2-146 which could be incorporated into this natural product [71] has been described by Perkins and coworkers (Scheme 2.33) [70, 72]. The conjugate addition/ Dieckmann-type cydization utilizing organocopper species as Michael donors afforded the enantiopure 2-145 in 68% yield. A further methylation of the (3-ketoester moiety in 2-145 followed by an elimination led to the desired cydohex-enone 2-146. 

Hetero Diels-Alder reactions using nitroalkenes followed by 1,3-dipolar cycloadditions provide a useful strategy for the construction of polycyclic heterocycles, which are found in natural products. Denmark has coined the term tandem [4+2]/[3+2] cycloaddition of nitroalkenes for this type of reaction. The tandem [4+2]/[3+2] cycloaddition can be classified into four families as shown in Scheme 8.31, where A and D mean an electron acceptor and electron donor, respectively.149 In general, electron-rich alkenes are favored as dienophiles in [4+2] cycloadditions, whereas electron-deficient alkenes are preferred as dipolarophiles in [3+2] cycloadditions. 

Scheme 3.52), whereas the corresponding acetyl-protected donor gave little product under the same reaction conditions [512], Additionally, the trichloroacetimidate method has also been applied to synthesize other natural products, such as bupres-tin A and B [517], macrophylloside D [518] and neomycin mimetics [519]. 

The scope of this cycloaddition reaction was very promising. Subsequently, removal of the CC-double bond and stereoselective functionalizations at positions 4 and 5 (carbohydrate numbering) was investigated for the synthesis of carbohydrates and related natural products, to provide C-3 branched carbohydrate derivatives (or C-4 heteroatom substituted derivatives after carbon/he-teroatom exchange reactions). However, the desired hydrogenation of such systems with various hydrogen donors has mainly resulted in low yields and/or side reactions due to the inherent stability of the formal CC-double bond (12., 15). ... 

Compounds in which two donor atoms are linked by a three-carbon chain undergo C-C bond cleavage readily. Well-known reactions are the retro-aldolization, retro Claisen, retro-Michael, and retro-Mannich reactions. Significant application of such processes to synthesis of complex natural products include approaches to caryophyllene [80], nootkatone [81], trihydroxydecipiadiene [82], hybridalactone [83], and mesembrine [84],... 

O-Glycosylation—that is, the condensation of a sugar derivative bearing an appropriate functional group (donor) with an appropriately protected other sugar derivative with a free hydroxyl group (acceptor)—is a crucial methodology in carbohydrate and natural product chemistry.744... 

Glycosyl fluorides, widely used reagents in carbohydrate and natural product chemistry,745 746 can be used to carry out stereoselective synthesis of glycosides with a catalytic amount (5 mol%) of triflic acid. The appropriately protected /3-D-glycosyl fluorides of glucose and galactose as donor molecules, when applied in dichloro-... 

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