Hmm, interesting problem. Here is one solution that lets you specify how many leading digits you want before the decimal point, before converting to char vector/string:
>> N = 1.45237E+6;
>> D = 3; % digits before decimal point
>> T = 6; % total digits
>> K = 10^(2-D+floor(log10(N)));
>> sprintf('%d%.*e',fix(N/K),T-D,mod(N,K))
ans = '145.237e+04'
Try it with more digits:
>> D = 5;
>> T = 6;
>> K = 10^(2-D+floor(log10(N)));
>> sprintf('%d%.*e',fix(N/K),T-D,mod(N,K))
ans = '14523.7e+02'
