Kinetics and mechanism of the N7-hydroxyalkylation of guanosine

The kinetics of the N₇-hydroxyalkylation of guanosine by the equally substituted epoxide, trans-2,3-epoxybutane, and the unequally substituted epoxides, 3-chloro-1,2-epoxypropane, and 1,2-epoxypropane in glacial acetic acid, have been measured by a spectrophotomeric method over the range 20-40°. Activation parameters have been determined. Comparative rates calculated from the ratios of second-order rate constants indicate that 1,2-epoxypropane reacts with guanosine about three times faster than does trans-2,3-epoxybutane and about two times faster than 3-chloro-1,2-epoxypropane. These results coupled with the results of a previous report on the structural analysis of the products from these reactions are consistent with a “push-pull” mechanism in which N₇ of guanosine reacts preferentially at the least substituted carbon of the epoxide with simultaneous transfer of a proton from acetic acid to the oxygen of the epoxide. The lower reactivities of trans-2,3-epoxybutane and 3-chloro-1,2-epoxypropane in comparison to that of 1,2-epoxypropane are discussed in terms of steric factors and electronic factors which determine the stability of the requisite transition state for a “push-pull” mechanism model.     

Acyclic stereoselection—13 : Aryl esters: reagents for threo-aldolization

Preformed Li enolates of hindered aryl esters condense with aldehydes to give predominantly threo aldols. The method has been explored with esters 3 (DMP propionate), 4 (BHT propionate), and 5 (DBHA propionate). DMP propionate reacts with benzaldehyde and α-unbranched aliphatic aldehydes to give threo:erythro ratios of about 6.5:1. However, with α-branched aliphatic aldehydes, ester 3 gives only threo-aldols. BHT propionate and DBHA propionate give only threo-aldols with all aldehydes studied. The DMP aldols may be converted into β-hydroxy acids by simple hydrolysis with KOH in aqueous methanol. BHT aldols cannot be hydrolyzed without retroaldolization. However, these aldols can be reduced to diastereomerically pure 1,3-diols. The DBHA aldols can be converted into β-hydroxy acids by a method involving oxidation with ceric ammonium nitrate (CAN) in aqueous acetonitrile. Threo-selectivity is also seen in the condensations of DMP butyrate (15), DBHA butyrate (16), DMP pentenoate (17), and BHT pentenoate (18). The approach has been utilized in a stereoselective synthesis of racemic methyl corynomycolate (30).     

Organic-free water for total organic carbon determination

Commercially available “organic-free” water is compared with water cleaned by ultraviolet (u.v.) radiation for applicability to total organic carbon determinations. The u.v.-treated water contains an organic carbon concentration less than the detection limit of the method used (0.056 mg C l^-1). The commercially processed water lacks the consistently low blank level required for use in TOC determinations.     

Some remarks on filtrations of hopf algebras

In this paper, the problem of nontrivial nearring multiplications on groups is closely investigated with a different approach. By using the notions of 5, 0-acts and the language of model theory, we give a detailed discussion on a large class of groups that possesses nontrivial nearring multiplications. In the last section, we apply our results to the theory of centralizer nearrings of a single endomorphism which is mainly due to the work of C. Maxson and K. Smith [28, 29]     

Moments of a Thue–Morse generating function

We study the moments of even order of the generating function

$$\prod\limits_{0 \leqslant r \leqslant n} {(1 - e({2^r}x))} $$

of the Thue–Morse sequence and present several conjectures related to these moments.