Smooth group action

Mono-substitution products of primary amines cannot easUy be prepared by direct action of the appropriate reagent for example, bromination of aniline yields largely the 2 4 6-tribomo derivative and nitration results in much oxidation. If, however, the amino group is protected as in acetanilide, smooth substitution occurs. Thus with bromine, />-bromoacetanilide is the main product the small quantity of the ortlio isomeride simultaneously formed can be easily eliminated by crystallisation. Hydrolysis of p-bromoacetanilide gives/ -bromoaniline ... 

Subjection of intermediate 16 to the action of 3 n aqueous HC1 in THF results in the formation of monocyclic lactol 14. In the presence of aqueous acid, the internal acetal grouping in intermediate 16 is hydrolyzed and lactol 14 is produced after the liberated secondary hydroxyl group attacks the terminal aldehyde carbonyl positioned five atoms away (see intermediate 15). Protection of the free aldehyde function in 14 with 1,1-dimethylhydrazine proceeds smoothly under dehydrating conditions and affords intermediate 13 in an overall yield of 72 %. 

The hydroxylation reaction, whose stereochemical course is controlled by the strong inherent preference for the formation of a cis-fused 5,5 ring system, creates a molecule which would appear to be well suited for an intramolecular etherification reaction to give ring E of ginkgolide B (1). Indeed, when a solution of 11 in methylene chloride is exposed to camphorsulfonic acid (CSA), a smooth cycli-zation reaction takes place to give intermediate 10 in an overall yield of 75% from 12. The action of CSA on 11 produces a transient oxonium ion at C-12 which is intercepted intramolecularly by the proximal hydroxyl group at C 4. 

The completion of the total synthesis only requires a few deprotection steps. It was gratifying to find that the final deprotections could be conducted smoothly and without compromising the newly introduced and potentially labile trisulfide residue. In particular, exposure of intermediate 101 to the action of HF pyridine results in the cleavage of all five triethylsilyl ethers, providing 102 in 90% yield (Scheme 23). Finally, hydrolytic cleavage of the ethylene ketal with aqueous para-toluenesulfonic acid in THF, followed by removal of the FMOC protecting group with diethylamine furnishes calicheamicin y (1) (see Scheme 24). Synthetic calicheami-cin y, produced in this manner, exhibited physical and spectroscopic properties identical to those of an authentic sample. 

Vasodilators are a group of dtugs, which relax the smooth muscle cells of the blood vessels and lead to an increased local tissue blood flow, a reduced arterial pressure and a reduced central venous pressure. Vasodilators reduce the cardiac pre-load as well as after-load and thereby reduce cardiac work. They are used in a variety of conditions including hypertension, cardiac failure and treatment/prevention of angina pectoris. Major groups are Ca2+-channel blockers (e.g. dihydropyridines), NO-donators (e.g. organic nitrates), K+-channel openers (minoxidil), phosphodiesterase inhibitors (e.g. sildenafil), Rho-kinase inhibitors (e.g. Y27632) or substances with unknown mechanism of action (e.g. hydralazine). Inhibitors of the... 

Ellingsrud and Strpmme have constructed the cell decomposition of P using the following results of [Bialynicki-Birula (1),(2)]. Let X be a smooth projective variety over k with an action of the multiplicative group Gm. We will denote this action by Let x X be a fixed point of this action. Let T x C Tx,x be the linear subspace on which all the weights of the induced action of Gm are positive. 

Let X be a smooth projective variety over k with an action of a torus H which has only finitely many fixed points. A one-parameter subgroup Gm —> H of H which does not lie in a finite set of given hyperplanes in the lattice of one-parameter groups of H will have the same fixed points as H. In future we call such a one-parameter group general . Thus the induced action of a general one-parameter group Gm — G has only finitely many fixed points on P. ... 

Here C(g) is the centralizer of g and [conjugacy classes of G. Hirzebruch and Hofer consider in particular the action of the symmetric group G(n) on the nth power Sn of a smooth projective surface 5 by permuting the factors. The quotient is the symmetric power S(" and ivn Slnl — is a canonical resolution of The canonical divisor Ks is invariant under the G(n) action. 

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