Marshall’s synthesis

Two additional syntheses of 12 employed organostannane and organobo-rane chemistry. Marshall s synthesis of 12 was based on chiral organostannane A as shown in Scheme 20 [30], while Oehlschlager s one was based on chiral organoborane A (Scheme 21) [31]. 

Scheme 6. Silver-catalyzed lactonization in Marshall s synthesis of (—)-Kallolide B (17).

Scheme 20 Marshall s synthesis of the C15-C24 subunit and completion of discodermolide...

Reaction of conjugated carbonyls with LiAlH(Ot-Bu)3 gives primarily 1,2-reduction, as in the quantitative reduction of 78 to 79 in Marshall s synthesis of globulol. goth sulfonate esters such as the mesylate group in 78, and halides are resistant to reduction with LiAlH(Of Bu)3. 

The presence of halogen atoms in a molecule is compatible with formation of an ylid and subsequent Wittig reaction. Carbon tetrabromide and triphenylphosphine have been used to give vinyl dibromide products, which are rapidly converted to the corresponding alkyne. Indeed, this is an important synthetic route to alkynes called the Corey-Fuchs procedure. The reaction of 511 with this reagent gave vinyl dibromide 512 in 77% yield as part of Marshall s synthesis of callystatin When 512 was treated with butyllithium... 

The above example is a clear black or white situation. Unless the dimethyl substituent is removed, the transform just cannot proceed. The grey areas cause just as much of a dilemma for the chemist, In Marshall s synthesis of isonootkatone two possible stereoisomers could have resulted. 

Use of these chiral allenyl metal reagents for the introduction of a propar-gyl group allows the synthesis of all four diastereomeric permutations of dipropionate subunits. The propargyl unit also provides a convenient handle for further elaboration, as demonstrated in Marshall s synthesis of the cytotoxic polyketide discodermolide (182, Scheme 5.30) [115]. Propargylation of aldehyde 177 with an allenylzinc species derived from mesylate 172 furnishes anti product 178 in 90 10 dr. The addition of allenylstannane 180 to aldehyde 179 affords the syn product 181 in an impressive 97% yield and >95 5 dr. 

How would you carry out the conversion (Iti) (17), the first steps of Marshall s bulnesol synthesis ... 

Yamada, T., Sartor, R.B., Marshall, S. and Grisham, M.B. (1992). A chronic model of distal colitis induced by bacterial cell wall polymers activation of leukocyte nitric oxide synthesis. Gastroenterology 102, A715. 

For a tandem Diels-Alder/fragmentation approach to the eleutherobin aglycone, Winkler et al. used Marshall s protocol in an early step of the synthesis. The building block 95 was prepared in that way (Scheme 15.24) [56]. 

Marshall s original but rather involved [10.10]betweenanene synthesis has been improved in their second approach which reportedly presented a sixfold improvement in overall yield55d). 

SCHEME 13.65 Marshall s allylation applied to the synthesis of destomic acid. Copyright 2006 by Taylor Francis Group LLC... 

Marshall s allylatlon applied to the synthesis of destomic acid... 

Marshall s chiral allenylmetal reagents have been utilized in double asymmetric reactions with chiral aldehydes for the synthesis of polypropionate natural products. All four dipropionate diastereomers are accessible from the reactions of chiral allenylmetal reagents with a-chiraI-y5-alkoxy aldehydes 97 (153, 158, 276, 277]. The BF3-OEt2-catalyzed addition of allylstannane (l )-218a to aldehyde 97a occurs in high yield and diastereoselectivity to give the xyn.syn-dipropionate 395, presumably through either the synclinal or antiperiplanar Felkin transition states 396 and 397 (Eq. (11.31)). 

The dienone (7.26) is the key for both the synthesis of (7.25) as above and in Marshall s total synthesis of p-vetivone and its stereochemistry is important in the subsequent steps of the syntheses. Hence, it is... 

There have been many syntheses of p-vetivone since that of Marshall and Johnson but perhaps the one that provides the greatest contrast is that of Gilbert Stork.710 One striking feature of Stork s synthesis is that it is essentially a two-pot synthesis. In one ingenious step, he builds the spirane system and does so with the correct stereochemistry. His reaction scheme, realised in collaboration with his co-workers Rick Danheiser and Bruce Ganem, is shown in Figure 7.13. 

To say that Stork achieved in two steps what Marshall and Johnson took 19, is something of an exaggeration since neither of Stork s starting materials are available off the shelf and therefore have to be synthesised. A fairer comparison can be made if Stork s synthesis is shown in full as in Figure 7.15 which shows how he made the two key components. 

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