Bishydroxylations

Benzodioxocin (14) can also be prepared starting from the dihydro derivative 17.14 Whereas an attempted bromination-didehydrobromination procedure was not successful, the required dehydrogenation was achieved by bromination with A -bromosuccinimide and treatment with triethylamine, by double bromination with iV-bromosuccinimide and debromination with zinc, or by pyrolysis of a diacetate derived from 17 by bishydroxylation and acetylation.14... 

The bishydroxylation of peripheral C —C double bonds of porphyrins, e.g. 6, with hydrogen peroxide under acidic conditions or with osmium(VlII) oxide yields the corresponding diols, e.g. 10, which on pinacol rearrangement are transformed into geminally dialkylated chlorins, e.g. 11.9,97... 

A major disadvantage of this procedure is the lack of regioselectivity of the bishydroxylation which can take place on each of the four pyrrole rings of the macrotetracycle so that, in addition to the monoadduct, two bisadducts and one trisadduct are formed. The subsequent pinacol rearrangement of the diols can also occur in two directions, leading to a complex mixture of additional constitutional isomers. 

The osmium(VIll) oxide bishydroxylation has been applied to a scries of unsymmetric porphyrins and chlorins bearing different electron-withdrawing or electron-donating groups which... 

Reduction of nitrone groups to hydroxylamines occurs readily with sodium borohydride (201). In particular, the action of NaBHj on l-hydroxy-3-imidazoline-3-oxides (268) leads to 1,3-dihydroxyimidazolidines (269), which on subsequent treatment with hydroxylamine hydrochloride afford 1,2-bishydroxyl-amines (270) (Scheme 2.99) (478). 

Bishydroxylation of an a-tetralone. The potassium f-butoxide catalyzed oxygenation (I, 921-923 2, 341) of the a-tetralone 1, gives minor amounts of the expected product 2. The major product (3) results from further hydroxylation of 2. The major product was shown to be a cis-i, 3-diol. It is reduced by lithium borohydride selectively to the triol 4. The triol has the same substitution pattern present in the A ring of certain anthracyclones.2... 

We have already seen the bishydroxylation using 0s04 and NMO this time lead tetraacetate is used to cleave the diol and yield the aldehyde. The phosphorane (Me0)2P(0)CHN2 is named the Gilbert-Seyferth reagent. 8 It basically behaves like the phosphoranes in the Horner-Wadsworth-Emmons reaction described above, except that the olefin subsequently loses nitrogen, creating the desired triple bond (also see Chapter 10). 

These reagents were first used for the bishydroxylation of olefins.406"408 It was later found that many of these reactions proceed via epoxides that undergo subsequent hydrolysis under the acidic conditions employed. Under basic or neutral conditions, these reagents can be used for the epoxidation of olefins409"414 ... 

It is noteworthy that both components are highly reactive and both required an in situ generation. The cycloadduct 7-56 was then subjected to a reductive ring contraction, acetylation, and a highly diastereoselective bishydroxylation to yield 7-58 which upon several protecting group transformations gave man-nostatin A 7-57 in enantiomerically pure form (Fig. 7-13). 

For much of this century, osmium tetroxide and potassium permanganate were alone in their ability to bishydroxylate alkenes. Such reactions are as... 

Acyloxy)alkenyl carbene complexes, synthesis, 223-224 Alkenes, bishydroxylation cycloreversion of rhenium diolates, 148-156... 

Beauveria sidfurescens has repeatedly been shown to be particularly suitable for hydroxyladons of a wide range of cyclic substrates, and much pioneering work was done by Fonken and coworkers, and is recorded in a number of patents held by Upjdm Co., two of which are pt cularly relevant.i Cyclohe-xylcyclohexane (16), for example, can be 4,4 -bishydroxylated by Beauveria su escens to give a 30% yield of the 4,4 -dihydroxy derivative (17). More recent work employing Beauveria sulfurescens or Cun-... 

The double bond is transformed into an aldehyde first. Bishydroxylation and oxidative cleavage creates the aldehyde. Aldehydes can be transformed into alkynes by a special phosphorane. 

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