Anti-base

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). 

J. Bjerrum (35) proposes to call Lewis acids for anti-bases, reserving the word acids for the species participating in Bronsted equilibria with solvated protons. Since a Brmsted acid hence contains hydrogen, many hydrogen-free anti-bases exist. Actually, Berzelius called species such as SO3 acids but for about 100 years, this word was reserved for compoimds containing hydrogen which can be replaced by metallic elements. J. Bjerrum then argues that all Lewis bases are also Bronsted bases. 

Hard anti-bases react preferentially with hard bases. 

Another simple example is the soft anti-base Tl+3 adding two electrons to Tl+ which is hardly a Lewis base though square-pyramidal green low-spin (5 = 1/2) Co(CN)f3 reacts54 with aqueous Tl+ to give (NC)sCoTlCo(CN)f5, where the distribution of oxidation states is debatable. 

In the case of polyatomic entities, the stereochemistry changes frequently drastically by adding two electrons to an anti-base. For instance, monomeric S03... 

H. Laatsch, AntiBase 2009 The Natural Compound Identifier, Wiley-VCH, Weinheim, Germany, 2009, www.wiley-vch.de/stmdata/anti-base.php, accessed 29 April 2009. 

When a prochiral acceptor (RiCH=A) and a prochiral donor (R2CH=D) react, the stereoisomers are labeled as either syn or anti based on the relative configurations of Ri and R2 when the Michael adduct is drawn in a zig-zag projection, as shown in Scheme 5.28. Using the Re/Si nomenclature and assuming that the CIP rank is A>Ri>H and D>R2>H, the syn adducts arise from Ik topicity and anti adducts arise from ul topicity. 

Purines -Syn and Pyrimidines -Anti base orientations (Figure bottom p. Ill)... 

The hydroxyketones that are provided by the aldolisation process can exist in two diastereoisomeric forms syn and anti) based on the transition-state geometry. 

Pearson suggested the principle that hard anti-bases (Lewis acids) pref-... 

Ikehara, M, Uesugi, S and Shida, T. (1980) Polynucleotides. LV. Synthesis and properties of dinucleoside monophosphates derived from adenine 8,2-S- and uracil 6,2 -0-cyclonucleosides Further support for the left-handed stacking of oligonucleotides giving high-anti base torsion angles. Chem Pharm Bull. 28, 189-197... 

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