A-adduct

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

At the same time, the reaction of 1,2,4-triazine 4-oxides 55 with the anion of chloromethyl phenyl sulfone affords 5-(l-chloro-l-phenylmethyl)-l,2,4-triazines 66. In this case, autoaromatization of the a -adducts proceeds by the deoxygenative... 

The lithium enolate of the 2(5//)-furanone 58 reacted with aldehydes to give a mixture of the y-adducts 154 and 155 together with the a-adduct 156, typically in a 1 1 6 ratio (Scheme 45) however, no significant selectivity was achieved (87TL985). 

When a solution of magnesium methoxide (prepared by the reaction of magnesium with methanol) is saturated with carbon dioxide, an active carboxylating agent, MMC, is produced. The reagent carboxylates substrates capable of enolization apparently by promoting formation of the magnesium chelate of the a-adduct. The reaction has been... 

Inversion of the regioselectivity in the addition of 2-butenylmagnesium chloride to aldehydes in favor of the a-adducts is caused by aluminum trichloride. A typical experiment is shown24 ... 

Lithiated (E)- and (Z)-l-(diphenylphosphinyl)-2-methyl-2-butenes undergo 1,4-addition to 2-cyclopentenone to mainly (E)-syn- and ( )-a /-adducts respectively2. 

Addition of lithiated Ar-(4-methylphenylsulfonyl)-S-phenyl-S -(2-propenyl)sulfoximine to acyclic enones gives exclusively 1,4-a-adducts in high diastereomeric purity. The configuration of the adduct (R2 = R3 = C6H5) has been determined by a single crystal X-ray structure determination27. 

Danishefsky s diene).46 The two donor substituents provide strong regiochemical control. The D-A adducts are trimethylsilyl enol ethers that can be readily hydrolyzed to ketones. The (3-methoxy group is often eliminated during hydrolysis, resulting in formation of cyclohexenones. 

Phenyl vinyl sulfoxide can serve as an acetylene equivalent. Its D-A adducts can undergo thermal elimination of benzenesulfenic acid. 

Vinylphosphonium salts are reactive as dienophiles as a result of the EWG character of the phosphonium substituent. The D-A adducts can be deprotonated to give ylides that undergo the Wittig reaction to introduce an exocyclic double bond. This sequence of reactions corresponds to a D-A reaction employing allene as the dienophile.71... 

Elimination of nitrogen from D-A adducts of certain heteroaromatic rings has been useful in syntheses of substituted aromatic compounds.315 Pyrazines, triazines, and tetrazines react with electron-rich dienophiles in inverse electron demand cycloadditions. The adducts then aromatize with loss of nitrogen and a dienophile substituent.316... 

Benzyne reacts with 7-dehydrocholesteryl methyl ether to form the ene products (110) and (111, X = H), while with tetrafluorobenzyne we isolated the anticipated cyclo-adduct (112) in addition to an ene product (111, X = F) 152>. We expected that less crowding would obtain in the transition states leading to cyclo-adducts from cholesta-2,4-diene, and obtained from a reaction with benzyne both the a- and 3-adducts (113, X = H) and (114). In a reaction with tetrafluorobenzyne we only obtained the a-adduct (113, X = F). 

We have also investigated the reactions of benzyne and tetrachlorobenzyne with the rings A/C cis-diene (117), both of which gave the a-adducts (118, X = H or Cl)... 

The diversity of the Ugi-MCR mainly arises from the large number of available acids and amines, which can be used in this transformation. A special case is the reaction of an aldehyde 9-26 and an isocyanide 9-28 with an a-amino acid 9-25 in a nucleophilic solvent HX 9-30 (Scheme 9.5). Again, initially an iminium ion 9-27 is formed, which leads to the a-adduct 9-29. This does not undergo a rearrangement as usual, but the solvent HX 9-30 attacks the lactone moiety. Such a process can be used for the synthesis of aminodicarboxylic acid derivatives such as 9-31 [3, 30],... 

Benzoporphyrin derivative, monoacid, ring A adduct BPD, VERTEPORFIN, VISUDYNE... 

Pure trans-2-chloro-3-pentene was used. The a-adduct is a mixture of trans. cis - 55 45. dNo cis isomer is formed. eThe stereoisomer ratio is trans cis = 92 8. 

Energies for the BPDE-N2(G) and -N6(a) adducts are measured relative to the 5 -orientation of the corresponding isomers in Tables VIII and IX, respectively. b See Figure 11. c See Figure 13. 

The stereoselected Cda conformation of the BPDE i(-) and Il(-) adducts to N6(a) were chosen for study in a reoriented complex with an externally bound pyrene moiety. In Figure 13, the adduct is shown in its optimum orientation in B-DNA with adenine after an anti - syn transformation for which the non-bonded contacts are poor, and with the normal anti base orientation with favorable contacts. The fit improves for the anti base as ax 30°. The orientation of the pyrene moiety is a(BPDE) =31° and the local helical axis of the DNA is oriented at y(DNA) = 15° Calculations were not performed with externally bound BPDE-DNA adducts to 06(G) and NU(C). Calculations of externally bound BPDE I(-)-N6(a) adducts with kinked DNA with ax + 30° yields an orientation a(BPDE) = 31° in good agreement with experimental results for the externally bound component (51). 

The energies reported in Table XIII for the externally bound forms are measured relative to that for the intercalative covalently bound form. Thus, the trans BPDE l(+)-N2(G) adduct is 10.1 kcal/ mole more stable and the trans BPDE II(-)-N6(a) adduct is 12.7 kcal/mole less stable in the externally bound form. Similarily, the trans BPDE Il(+)-N2(G) adduct is -13.8 kcal/mole more stable and the trans BPDE I(-)-N6(a) adduct is 7 5 kcal/mole less stable. Therefore, site IQ (intercalative covalent) which is favored by the i(-) isomer (5l) may be due to n6(a) and NH(c) adduct formation, specifically trans addition. 

Figure 13- An externally bound BPDE Il(-)-N6(A) adduct with the pyrene moiety placed in the major groove. The receptor sites are (upper) B-DNA except for an anti syn rotation by l80° about the glycosidic bond of A, and (lower) DNA with an ax = 30° kink.

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