My thesis project is divided into two parts viz. parts a and b. part a deals with a transition-metal-free biaryl-coupling approach to the total syntheses of benzo[c]phenanthdridine alkaloids. benzo[c]phenanthridine alkaloids are important class of alkaloids having interesting biological activities such as anti-microbial, anti-viral properties and few members of this class are found to be potential antitumor drugs inhibiting dna topoisomerase i. this first part of my research, mainly deals with a kotbu-promoted intramolecular homolytic aromatic substitution (ihas) with the aid of a catalytic amount of bidentate organic ligands like 1,10-phenanthroline, dmeda, 2, 2’-bipyridine. interestingly, it was observed that the method also works just in the presence of kotbu, without the use of organic molecule as ligand. utilizing the aforementioned strategy, total syntheses of dihydronitidine, dihydrochelerythrine, and dihydroavicine have been accomplished. further exploration of substrates having different protecting groups on nitrogen, we have synthesized benzo[c]phenanthridine alkaloids viz. nornitidine, and norchelerythrine. to show the versatility, the strategy was further utilized in the synthesis of amaryllidaceae alkaloids viz. oxoassoanine, anhydrolycorinone, 5,6-dihydrobicolorine, and trispheridine. a tentative mechanism has been proposed to understand the sense of biaryl-coupling reaction via ihas process. part b of this thesis entitled "synthetic approaches to the dimeric pyrroloindoline alkaloids via organocatalytic aldol reaction" deals with construction of all-carbon quaternary stereocenters via a bifunctional thio-urea catalyzed enantioselective aldol reaction and its application to the synthetic approaches to the alkaloids sharing 3a,3a'-bispyrollindino[2,3-b]-indoline unit. retrosynthetically, we envisioned that 3a,3a'-bispyrollindino[2,3-b]-indoline alkaloids can be synthesized from c2-symmetric advanced intermediate (r,r)-hydroxyl methylated bis-2-oxindole, which in turn can be synthesized from enantioselective aldol reactions of (±)-and meso-bis-n-boc protected dihydro isoindigo via a dynamic kinetic asymmetric transformation. however, in practice, we found that aldol reaction of bis-n-boc protected dihydro isoindigo using paraformaldehyde in dichloromethane at 30 ºc (optimized condition) in the presence of cinchonine derived tu catalyst, l1 provided only enantioenriched spiro-lactone compound (tert-butyl (3s)-4-(2-((tert-butoxycarbonyl)amino)phenyl)-2',5-dioxo-4,5-dihydro-2h-spiro[furan-3,3'-indoline]-1'-carboxylate) in 93% ee with a poor ~1:1 dr. we also envisioned that, after the first introduction of hydroxymethyl group via aldol reaction, it immediately reacts with electron-deficient amide functionality to form spiro-fused lactone. after that a second diastereoselective aldol with paraformaldehyde was carried out in the presence of 10 mol% of l1 in ch2cl2 at 25 ºc to afford pseudo-c2-symmetric corresponding hydroxymethylated product in 85% yield and ~12:1 dr with 91% ee (essentially through match-match pair of stereochemistry of starting material and l1). similarly, a second diastereoselective aldol of a was performed in the presence of 10 mol% of pseudoenantiomeric ligand of l1 i.e. l2 in ch2cl2 at 25 ºc to afford enantioenriched bis-2-oxindoles in 81% yield with ~10:1 dr and 91% ee, which on subsequent treatment with tfa afforded corresponding meso-product in 77% yield with ~10:1 dr. due to interesting biological activities of pyrroloindoline alkaloids we thought of developing a unified strategy for the synthesis of pyrroloindoline alkaloids as well as spiro-pyrroloindoline alkaloids. therefore, it was decided to use substrates of type 3-alkylated n-boc protected 2-oxindoles having. gratifyingly, it was found that, under the optimized condition (10 mol% of l1 in acetonitrile at 5 ºc), a variety of enantioenriched 2-oxindoles having all-carbon quaternary stereocenter can be synthesized in high yields with excellent ee's. it was then decided to use enantioenriched aldol product, (s)-3-(2-(((benzyloxy)carbonyl)(methyl)amino)ethyl)-3-(hydroxymethyl)-2-oxoindoline-1-carboxylate as potential intermediate, from where total syntheses of spiro-pyrroloindoline alkaloids, (+)-coerulescine and (+)-horsfiline have been completed in 2-3 steps. total syntheses of (-)-deoxyeseroline and -)-esermethole have been completed in 2-3 steps from same enatioenriched potential intermediate. further, first enantioselective syntheses of prenylated pyrroloindole alkaloids, (-)-pseudophrynamines 272a and 270 have also been accomplished from entio-pure cyclotryptamine intermediate.