  There is mounting evidence that the composition and structure of planetary
systems are intimately linked to their birth environments. During the past
decade, several spectral surveys probed the chemistry of the earliest stages of
star formation and of late planet-forming disks. However, very little is known
about the chemistry of intermediate protostellar stages, i.e. Class I Young
Stellar Objects (YSOs), where planet formation may have already begun. We
present here the first results of a 3mm spectral survey performed with the
IRAM-30m telescope to investigate the chemistry of a sample of seven Class I
YSOs located in the Taurus star-forming region. These sources were selected to
embrace the wide diversity identified for low-mass protostellar envelope and
disk systems. We present detections and upper limits of thirteen small ($N_{\rm
atoms}\leq3$) C, N, O, and S carriers - namely CO, HCO$^+$, HCN, HNC, CN,
N$_2$H$^+$, C$_2$H, CS, SO, HCS$^+$, C$_2$S, SO$_2$, OCS - and some of their D,
$^{13}$C, $^{15}$N, $^{18}$O, $^{17}$O, and $^{34}$S isotopologues. Together,
these species provide constraints on gas-phase C/N/O ratios, D- and
$^{15}$N-fractionation, source temperature and UV exposure, as well as the
overall S-chemistry. We find substantial evidence of chemical differentiation
among our source sample, some of which can be traced back to Class I physical
parameters, such as the disk-to-envelope mass ratio (proxy for Class I
evolutionary stage), the source luminosity, and the UV-field strength. Overall,
these first results allow us to start investigating the astrochemistry of Class
I objects, however, interferometric observations are needed to differentiate
envelope versus disk chemistry.
