Convergent preparation

Axial addition to oxocarbenium ions derived from 1,3-dioxanes provides protected a f/-l,3-diols. Our group has developed 4-acetoxy-1,3-dioxanes as oxocarbenium ion precursors. This general strategy for the convergent preparation of anfz-l,3-diols complements cyanohydrin acetonide methodology, which gives access to sy -l,3-diol synthons (Sect. 2). 

Li J, Jeong S, Esser L, Harran PG (2001) Total Synthesis of Nominal Diazonamides - Part 1 Convergent Preparation of the Structure Proposed for (-)-Diazonamide A. Angew Chem Int Ed 40 4765... 

Scheme 5.38. The convergent preparation of polysiloxane dendrimers also employing silicon as a branching center.

Kamikawa used two different Pd-catalyzed C-N bond-forming reactions for the convergent preparation of several phenazines [93]. After intermolecular coupling was achieved, the nitro group was reduced. Subsequent Pd-catalyzed cyclization afforded the desired product in 80% yield over three steps, Eq. (175). 

Tietze et al. emphasized the usefulness of chiral trimethylsilyl ethers of readily accessible amino alcohol derivatives in allylation of aldehydes and ketones [43]. As a consequence, careful design of the norpseudoephedrine derivatives and proper choice of silicon Lewis acids have led to the convergent preparation of enantiomerically enriched secondary and tertiary homoallylic alcohols in high yields (Sch. 12) [43a], It should be noted that the configuration of the newly formed stereogenic center of the secondary homoallylic alcohols is the opposite of that in the allylation of ketones [43c], They also described in detail mechanistic studies of the above allylation reaction by use of and NMR. 

In the following section, a variety of convergently prepared dendrimers will be discussed in the general context of their applications. Given the focus of this review, such a classification is quite appropriate since it is the application that dictates the design of the dendrimer and hence its synthesis. 

An alternative approach developed by Schluter and co-workers involves the attachment of convergently prepared dendrons onto a preformed, functional linear polymer. This was first attempted by the Williamson ether couplings between dendritic bromides and pendant hydroxyl groups on poly([ 1.1.1]-propellane)412 and polyp-phenylene).413 Nearly quan-... 

Dendritic layer copolymers have also been prepared via the hypercore approach,87 and by divergent growth from the periphery of convergently prepared dendrimers,446 or dendronized polymers.398 This layered dendritic architecture has been used by a number of groups to probe the nature of chirality. 

Another example of an enantioconvergent transformation of racemic epoxides was reported by Faber and coworkers who employed a chemoenzymatic approach for the synthesis of optically enriched diols. They used an (S)-selective epoxide hydrolase from Nocardia sp. in connection with an add catalyzed hydrolytic step to convergently prepare (S)-diols 41 in good to excellent yields (71-98%) with enantiomeric excess values up to 99% (Scheme 9.13) [75]. 

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