Addition sequences

An important method for construction of functionalized 3-alkyl substituents involves introduction of a nucleophilic carbon synthon by displacement of an a-substituent. This corresponds to formation of a benzylic bond but the ability of the indole ring to act as an electron donor strongly influences the reaction pattern. Under many conditions displacement takes place by an elimination-addition sequence[l]. Substituents that are normally poor leaving groups, e.g. alkoxy or dialkylamino, exhibit a convenient level of reactivity. Conversely, the 3-(halomethyl)indoles are too reactive to be synthetically useful unless stabilized by a ring EW substituent. 3-(Dimethylaminomethyl)indoles (gramine derivatives) prepared by Mannich reactions or the derived quaternary salts are often the preferred starting material for the nucleophilic substitution reactions. 

Effect of Reagent Addition Sequence on Isobutylene Polymerization. 92... 

It was postulated that the rate decreased as the basicity of the halogen decreased and/or steric compression increased in f-BuX, and as the polarizability of halogen in MeX increased. The objective of the present research was to extend this model study to isobutylene polymerization systems, in particular to investigate the effect of reagent addition sequence and that of the nature of the halogen in f-BuX and MeX on the polymerization rate and PIB yield using Me3 Al coinitiator. 

Table 2. Effect of reagent addition sequence on isobutylene polymerization...

Time °C Time min. Yield8 % Addition sequence Comments... 

Using the f-BuX/Me3Al/MeX system, a preferred reagent addition sequence has been found to be /-C4Hg/MeX/Me3 Al/t-BuX. This sequence has been used in these investigations. Based on polymerization rates at —40 °C, overall polymer yields, floor temperature and initiator efficiencies at —40 °C, overall initiator reactivity is found to decrease as f-BuCl > f-BuBr > t-BuI = 0 and initiator reactivity is dependent on solvent as MeCl > MeBr > Mel = 0. Similarity of reactivity sequences in isobutylene polymerization and in cationic model initiation and termination studies13) suggest that initiator reactivities are determined by the rate of initiation, Rj. 

Besides the mechanisms already discussed, another mechanism, involving an elimination-addition sequence, has been observed in vinylic systems (a similar mechanism is known for aromatic substrates, p. 854). An example of a reaction involving this mechanism is the reaction of 1,2-dichloroethane with ArS and OEt to produce 80. The mechanism may be formulated as... 

The steps are the same as in the addition-elimination mechanism, but in reverse order. Evidence for this sequence is as follows (1) The reaction does not proceed without ethoxide ion, and the rate is dependent on the concentration of this ion and not on that of ArS. (2) Under the same reaction conditions, chloroacetylene gave 83 and 80. (3) Compound 83, treated with ArS, gave no reaction but, when EtO was added, 80 was obtained. It is interesting that the elimination-addition mechanism has even been shown to occur in five- and six-membered cyclic systems, where triple bonds are greatly strained. Note that both the addition-elimination and elimination-addition sequences, as shown above, lead to overall retention of configuration, since in each case both addition and elimination are anti. 

The elimination-addition sequence has also been demonstrated for certain... 

Of course, we have seen (p. 430) that SnI reactions at vinylic substrates can be accelerated by a substituents that stabilize that cation, and that reactions by the tetrahedral mechanism can be accelerated by P substituents that stabilize the carbanion. Also, reactions at vinylic substrates can in certain cases proceed by addition-elimination or elimination-addition sequences (pp. 428, 430). 

Underlined sequences indicate amino acid sequences used for the generation of degenerate primers. Bracketed question marks represent blank cycles from the Edman degradation reaction. Additional sequence was obtained after blank cycles in all cases except the Glu-C-1 and Glu-C-2 peptides. 

There are instances where the reflux temperature decreases with conversion of the reagents, and exothermic reactions may even lead to hazardous runaway conditions. In such cases it may become necessary to optimize an addition sequence of reagents to avoid unwanted exotherms. Anderson et al. (1997) have given a practical example for an important intermediate in making the drug monopril. 

It is important to remember that the yield and reproducibility of hydrozircona-tions and the subsequent transformations depend on the presence of ionic impurities in 1 which often hamper the desired transformations. For example, in situ preparations of 1 with liEtjBH and BuMgCl were not appropriate for hydrozircona-tion/copper-catalyzed conjugate addition sequences otherwise preformed 1 is well adapted for these protocols. 

The regiochemistry of the Heck reaction is determined by the competitive removal of the (3-proton in the elimination step. Mixtures are usually obtained if more than one type of (3-hydrogen is present. Often there is also double-bond migration that occurs by reversible Pd-H elimination-addition sequences. For example, the reaction of cyclopentene with bromobenzene leads to all three possible double-bond isomers.146... 

Reference codes are those used in the TRC SOURCE database. A reference code consists of the year prior to 1900, or the last two digits of the year after 1899, the first three letters of the first author, the first three letters of the second author. An additional sequence number is used when more than one reference in the database has an identical code. 

An additional sequence element within the mouse ZP3 promoter has been identified as the binding site of an oocyte-specific protein called OSP-1 (Schickler et al., 1992), although the possible effect of mutations of this element on transcriptional activity has not been established. Nonetheless, it is interesting that a similar sequence (GATGA) is present 40 nucleotides upstream of the c-mos transcription initiation site (Fig. 5), although deletion of this element had no discernible effect on the activity of the c-mos promoter in microinjected oocytes (Pal et al., 1991). 

The most efficient strategy may be to use the topoisomerase cloning method in conjunction with the Crs-lox or X recombination systems. In this way, PCR products can be efficiently inserted into the Univector or Entry Clone with minimal additional sequences added to PCR primers. Once the genes are inserted into the starting vectors they can rapidly be converted to functional vectors using either Cre-lox or X recombination. 

As the last example of this section, a conjugate addition sequence of Ikeda and coworkers will be discussed, in which a somewhat unusual Michael system 2-26 was used (Scheme 2.6) [11]. Reactions of 1-nitro-l-cyclohexene (2-26) and 4-hy-droxybutynoates 2-27a-c in the presence of the base KOtBu gave R/Z-mixtures of substituted octahydrobenzofurans 2-28a-c in excellent to good yield. 

Scheme 2.56. Aldol/Prins-type/addition sequence.

A Knoevenagel condensation/Michael addition sequence has been reported by Barbas III and coworkers (Scheme 2.70) [158] using benzaldehyde, diethyl malonate, and acetone in the presence of the chiral amine (S)-l-(2-pyrrolidinyl-methyl)-pyrrolidine (2-301). As the final product the substituted malonate 2-302 was isolated in 52% yield with 49% ee. 

The research groups of Mariano and West developed a photoinduced electrocydi-zation/nucleophilic addition sequence. Thus, irradiation of N-alkylpyridinium perchlorates as 5-19 in an aqueous solution led to the aziridine cations 5-20, which react in a nucleophilic addition with OH to give the isolable azabicyclo[3.1.0]hex-2-enols 5-21. These can be further transformed by a nucleophilic ring-opening of the aziridine moiety under acidic conditions to lead to useful unsymmetrically trans,trans-trisubstituted cyclopentenes 5-22 (Scheme 5.5) [10]. 

Another method that yields quinolizidine derivatives by creation of two ct-bonds from acyclic precursors is based on a domino process involving a sequence of a double N-deprotection and a double intramolecular Michael addition sequence of reactions, as summarized in Scheme 75 <2002TL6505>. 

Bose has described reactions between acid chlorides 214 and Schiff bases 215 where the stereoselectivity depends on the order of addition of the reagents (Scheme 9.68) [117]. When the condensation was conducted by a normal addition sequence (i.e. acid chloride last), only the cis /1-lactam (216b) was formed. However,... 

It seems that no general mechanistic description fits all these experiments. Some of the reactions proceed via an addition-elimination mechanism, while in others the primary step is electron transfer from the arene with formation of a radical cation. This second mechanism is then very similar to the electrochemical anodic substitution/addition sequence. 

Only the octahedral W(CO)6 final product of the co-addition sequence has a VSEPR-like structure (but the three additional lone pairs on W would also lead to a different prediction for this case if strict VSEPR reasoning were applied). The intermediate 12e W(CO)3, 14e W(CO)4, and 16e W(CO)s molecules are all perfectly stable as isolated species. However, it is clear that, in the presence of excess CO, only the 18e W(CO)6 species will remain as a final product, and the 18e configuration will therefore appear to exhibit special stability. ... 

The conserved residues identified as 50 were chosen from a sequence alignment analysis. There are more than 100 residues among a set of 51 mammalian sodium-dependent NTs of known function that are 100% conserved (alignment not shown). To decide which residue is the most appropriate reference residue for each of the TMs, we searched for additional sequences with similarity to the mammalian NTs, and investigated the decrease in conservation for alignments of increasing size. 

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